What's the difference in 11b and 11c? Which is better?

The 11 Series MOS's are so close it is why they share the same number but end in a different letter. So it is personal preference. If you really want 11B over 11C have a one on one with your Drill Sergeant in Infantry OSUT and he may or may not help you out, depending on how they select these days. Niether MOS is really better than the other they just have different primary tasks. The secondary tasks are the same for both. So your training on Mortars will be your only difference with 11C vs 11B. While 11B is doing advanced patrolling and advanced movement tactics, you will be trained on mortars and you will get the advanced patrolling and advanced movement tactics at your unit instead of Infantry OSUT. Typically that is how they used to structure the training. So as a 11C you will get all the 11B training but the 11B will not get the 11C training.

11B/C basically the same, but I never wanted to be a mortar man, stay away from it

11B: Infantry - close to the bullets
11C: Mortars - not as close to the bullets

But always remember that crew served weapons will be particularly targeted when encountered.
As infantry I would give targeting priorities as follows
Artillery and air power, although usually the infantry is too far off to engage these threats directly
Anti Armor, Mortars & Machine guns, whichever is closer to hand
Grenadiers
Riflemen

Of course I was never a tactician, just a grunt, and thanks be to God never had to put any of this to the test.