running on a treadmill.

I think both are good and work your body differently so you could alternate. one week raise your speed, the next week lower your speed back down and do a higher incline. I only increase speed not incline as the treadmill I use BANGS when running faster than 5.0 if it is on an incline.

You should alternate between speed workouts and incline. Try to strike a balance. You can do it on alternating days or just within the same session. Even adding a 3 % incline can really rev up your workout. Start small and gradually increase the incline when you are able. Running hills is a VERY effective calorie burn and varying your workouts will also increase calories burned. If you do the same workout all the time, your body becomes very efficient at that activity. In other words, your body starts using LESS calories to do the same workout. And remember don't expect to be able to run the same speed w/ an incline.

Not sure what you mean by "beneficial" but like others have said it is good to alternate so your body doesn't get used to one thing and always using the same muscles. However, if it is fat loss you're looking for the best thing to do is interval training. Walk (No more than 4 mph) for 1 1/2 to 2 minutes and then sprint (8-12 mph increase as you get better) from 30 seconds to 1 minute. I usually only do 30 minutes of interval training because I am training for a marathon and already run a few miles before doing this but if you aren't doing another form of cardio I would increase to 45-60 min of interval training, you should be good and tired at the end.