Which do you do and why? Both are my favorite exercises. Lately, I've been doing chin-ups at the beginning of my workout, and curls at the end. Chin-ups workout my back and stomach too, but I can do heavier resistance with curls. I'm on the fence as to which I should stick to.

I really don't understand why you'd exclude one or the other. Keep your chin ups as they're an excellent movement. Throw in a couple of isometric holds on your chins at the last 1-2 reps for stimulation a bit more of the biceps. Hit your bicep variation movements at the end of your workout. And make sure you're working your triceps as much as your biceps.

I do chins and curls.
If I had to choose, i'd pick chins just because its a more intense exercise. Though i enjoy doing curls more, I love the feeling of that pump and when your veins are poppin out when your at the height of the curl.

As arnold used to say, it feels like cumming.

you can curl more than you can chin

Do both, there is no reason to choose one or the other.

Neither. Pull ups and rows hit my biceps

Yeah, I don't really see any point in wasting my time doing an exercise like curls that only hit a very small number of muscles. Chin-ups work a lot more muscles than curls. After my whole upper body workout, my biceps get worked in most of the exercises, there's no need to add curls on top of that.

My arms are disproportional to my weight (I have 13.5" arms, but I only weigh 142lbs) so pull-ups and chin-ups are very easy for me. I think I should invest in a weighted belt or vest.

Absolutely. If you're a male in good shape you probably need to do weighted pull-ups or chins. "dip" belts don't cost that much.

I do chins and curls, i put down curls on the poll as they are the iso that i directly do for the biceps.

Any pull exercise will work the biceps in some way.

I usually do chin ups and dips. I would do pull ups, but they hurt like crap so I stopped since I didn't find it fun. For chin ups, I do roughly 10-12 per day, and dips I do about 5-6. I usually start off with 5 chin ups with arms about shoulder length, and then I go about my exercises, and end it with a stretch, and chin ups on the bars sticking out, and dips. It feels great after doing cardio such as running or what not.