I am with @kristinegift , I carry a 10 oz handheld for my marathons for those very reasons she stated (except I only put water in it). But I see a lot of people with hydration belts at races. If that's what you are used to, I would stick with it.

To hop in on the fuel belt/hand held debate:
I used a fuel belt for about a year of training and two half marathons. I really liked that it gave me space to store my phone and keys and that I was able to keep my hands free. Saw a lot of other people wearing them at races too. It worked really well for me for a while, except I found that I had to slow down a lot in order to take the bottles out of the little holders and put them back again. This didn't bother me when I was running 2+ hour half marathons and walking through some of the water stations any ways. Once I got faster though, I started to care about the 5-10 seconds I lost everytime I had to mess with the bottles, which is why I switched to a hand held about a year ago. Still not crazy about having to carry it, but now I don't have to slow down at all to hydrate. I recently bought a hydration vest that I'm planning to test out for some of my long training runs this summer so I guess I'll have to re-update you then

I wear a hydration belt every time I run more than 2.99 miles. Which means I only run without it 2 or 3 times a year. My second 5k race I didn't wear my belt and I tried drinking from a cup on the run and wound up choking and coughing, so know I always take my own water. This has the added advantage of not being tied to where and how often the organizers decide to place drink/aid stations.

I have an Amphipod RunLite Stretch 4Plus(?)(they have changed their line up and don't actually sell what I have anymore). The bottles I have are not their current XTech bottles, which self center as you snap them into the "docking base", but the 1st(?) generation bottles where a ridge on the arms of the base snap into a grove that runs part way along either side of the bottle. This means that the grove has to be aligned properly to the base to properly snap in and stay put. Once I determined that it would take a little effort/skill to undock and redock the bottles I made a point of practicing it before each run while I was doing my warmup routine until it became second nature.

Using a hydration belt bottles and pouches or a handheld bottle is just like any other component of race readiness, you need to train the techniques ahead of time so that you don't have to think about it during a race, you just do it.

I ran a marathon two weeks ago and at the start it was 30 degrees and it warmed up into the 50s by noon. I wore a jacket and 5 miles in I took my arms out of the jacket, leaving the bottom tucked under my hydration belt, and knotted the sleeves so the top would not flop around or drag on the ground. This basically meant every time I accessed my bottles or pouches that I was doing it by feel under my jacket, but I didn't have to slow down or stop because it is all automatic for me at this point.

Whether you use a handheld or a belt is up to you, but make sure that you get some practice loaded up the way you will be at the start of your race. Even if it is only a 4 or 5 mile run, load up your 8 gels, and 16 oz.(?) of water, electrolyte tabs, etc.; and go for your run. If you are going to use your own electrolyte tabs, then start with plain water and add a tab a few miles in on the run. Practice getting your gels out of your various stashes while running and putting them back. What are you going to do with your empty gels? I put my empties in a snack size ziplock bag that I keep in a pouch or pocket, so that might be another thing to practice. If you are using a handheld then on your 12 mile run practice actually consuming a couple gels or chomps to make sure that you can manage opening the package and consuming the contents with a bottle in one hand.

You want to make sure you can do these different tasks without disrupting your running and that where you are stashing your supplies will not interfere with your running or cause irritation.