It is a little early for me to start worrying (1st race isn’t until May) but I figured I would ask since I have both now.
What would be more buoyant a full sleeved wetsuit or a sleeveless one? I am not worried about overheating in warm water, just which one floats better
full.
more neoprene equals more bouyancy. A full will be more bouyant. The question is, will it be faster?
full is faster for sure.
A full sleeve suit will be faster and warmer than a short sleeve one.
Many shy away from full suits after a bad first experience with a full suit and they think they will be better off in a short sleeve suit. Typically one, two or all of the three key things to great wetsuit fit is off with these folks:
They may have been in the wrong brand of suit - different brands fit differently
They may have been in the wrong size for them - getting the right size is often easier said than done. It helps when there is a direct, linear relationship between a companies size chart, what the suit says and what the persons body shape size is.
The suit needs to be fitted to a person properly - Again something many over look. I have often been to events and heard/seen people say their wetsuit sucks, that it’s really uncomfortable. I have then made a couple of quick key adjustments and then they go “Wow”, this suit feels great - and this can even be the case with non- Nineteen suits! 
Get the three above things right, and I assure you that you will be better off in a full suit.
Hope this helps.
I’ve wondered the same thing but perhaps for different reasons. My legs sink badly and I can’t strike a horizontal well. If I had a sleeveless suit, would that in fact better balance my upper vs lower body bouyancy? If so, I’d think this might offset the drag reduction benefits of the neoprene that I’d lose on the arms.