Rowing machine vs. treadmill

Neither will probably make much difference in terms of arm definition.
The amount of calories you will burn off depends on the settings and how much effort you put into it.
I'd go with a treadmill as it uses your whole body and doesn't impact your joints.

If you go for the rowing machine, make sure to be fully educated about how to use it. Improper posture leads to injury much faster on the rowing machine as compared to a treadmill.

No form of exercise is inherently superior to all other forms of exercise. The effects of exercise training come primarily from HOW you do the exercise, not WHAT you do.

Rowing is as good a form of exercise as any other. Like all forms of exercise, it has its pluses and minuses. The main caveat in recommending rowing as an exercise to a non-rower is that rowing is a very specific exercise movement that has a somewhat longer learning curve than some other forms of exercise (e.g. running, walking, elliptical). So you need to have some confidence that you will enjoy the movement and be willing to learn the proper technique.

If you have tried it, or are confident that you will enjoy it, then by all means go for it. If not, the main exercise will be an exercise in frustration.

One cannot accomplish strength training and cardiovascular conditioning simultaneously doing the same movement. So any claims that one modality will result in "more strength training" than all others is false. The only "strength increases" that can occur during cardiovascular training are the limited initial adaptation to the movement. For someone who has done no resistance training, those initial gains might be noticeable, but they will quickly plateau--and one will not gain appreciable amounts of muscle doing cardiovascular exercise of any type.

Calorie burn is based on intensity of effort, not on the amount of muscle mass involved in the movement. In studies of Olympic-caliber athletes, rowers had an average VO2 max that was lower than cross-country skiers, distance runners, speed skaters, and cyclists (although they did score higher than weight lifters). This, however, could be a result of self-selection rather than any inherent limitations in rowing. (Not to mention that competitive rowers tend to weigh noticeably more than the most of the other types of athletes mentioned).

The amount of EPOC generated with exercise is determined by intensity and duration of the exercise, not on any particular modality, so rowing confers no advantage in this area either.

If you do decide to get a rowing machine, I would enthusiastically recommend that you spend the extra $$ on a Concept 2 ergometer. The machine itself is not only excellent, but there is a strong community centered around the "erg" and you will find lots of training resources.

I would choose the Concept2 over the treadmill any day...
not only because I love rowing but also because one can jog-run pretty much anywhere....anytime..(okay not in the middle of a tsunami of course but..) but rowing out is more complicated- you need a lake or a river. then you need a boatshed, and a boat..and gear..and lifevest....the indoor rower makes it soooooo easy and one can get a pretty decent overall body workout on the indoor rower: great for your legs, your core and for the arm...
I wish I could purchase an indoor rower for my balcony but I live in a complex and have to get owner's corporation approval.

A lot of people buy machines that become coat racks, when the thrill wears off in couple of months. I would say forget about which burns more calories and focus on which one you don't mind doing month after month. Some women like rowing, but most women at my gym prefer the treadmill. If you've never rowed, get a day pass at a local gym and try one out for half an hour. First watch the videos on Concept2's website on how to row properly.

If you both aren't following a strength training program, there's another new year's resolution to consider.

sorry didnt see this one before I posted

I would choose the Concept2 over the treadmill any day...
not only because I love rowing but also because one can jog-run pretty much anywhere....anytime..(okay not in the middle of a tsunami of course but..) but rowing out is more complicated- you need a lake or a river. then you need a boatshed, and a boat..and gear..and lifevest....the indoor rower makes it soooooo easy and one can get a pretty decent overall body workout on the indoor rower: great for your legs, your core and for the arm...
I wish I could purchase an indoor rower for my balcony but I live in a complex and have to get owner's corporation approval.

This is pretty much my logic, as well.

I actually own both a rowing machine and treadmill, but if I could only have one, it would be a rowing machine because I can go outside and run. Obtaining a boat and a body of water upon which to exercise is a little more difficult (even back when I was living five minutes away from the coast). Plus, I do like that it incorporates more muscle groups across my whole body than just running and it doesn't bother my joints as much, either. The main reason I have a treadmill, too, is that I have to pass a run test for my job, so I have it on hand in case something happens that prevents me from going outside to run.

Neither will probably make much difference in terms of arm definition.
The amount of calories you will burn off depends on the settings and how much effort you put into it.
I'd go with a treadmill as it uses your whole body and doesn't impact your joints.

How can the treadmill not impact your joints?

I use both the treadmill and the rower. Because of joint problems I have to take breaks from the treadmill due to the pounding on my hips and knees. So far the rower...as long as I keep proper form...has never affected me since I am seated.

Both have their good and bad points depending on your body condition. I love to row...I can get in a rhythm and zone out for a while.

To start out the new year with a bang, my husband and I are either going to invest in a treadmill or a rowing machine. He would prefer rowing machine. I have a serious lack of arm definition (re: flab), but I also want the best calorie burn possible. Please discuss.

A rowing machine is a whole body exercise, that gives a degree of resistance training as well as a CV effect. That said the rate of calorie burn is much lower, and it's more challenging to lift the heart rate up. With a rowing machine you can complement that by going out running in the real world rather than staring at a wall for an hour at a time. The rowing machine is much lower impact than running, even on a treadmill, so you can train more frequently using it, with lower injury risk.

Personally I use a water rower and run.

and one more thing....if you get a Concept2 rower it is most likely to have a built in game!
You can play a "big fish eats small fish but watch out as a shark might eat you" game whilst rowing
it is not a high tech one but it is still something Things like that did not exist when I was rowing as a kid

I've had both but didn't keep the rower long because I sit for work all day and found I missed being upright for my exercise. Also the treadmill feels more versatile because I can use it at walking pace and read or work on the computer.