I've heard a lot of good things about the Condor Modular vest, it's one size fits all but it has shoulder straps for readjusting if you need to. The guys down at Modern Intel are going to be your best bet, they have reviews of everything and a shop
Well with what I've seen of .50 calibre paintballs, there aren't really very many benefits. So they weigh a little less, and have a softer impact than a regular .68 ball, and they increase efficiency as it takes less air to propel a smaller ball. Other than that, increased accuracy and the like have yet to be proven in my eyes. Now I'm no physicist, I'm not even an armchair physicist, so if anyone else has an opinion on fifty cal theirs is more valid than mine so please correct my thinking if I'm wrong. It seems unlikely that PunkWorks hasn't compared the two already, maybe you could ask them about pros and cons of it?