I've converted all my saws to 3/8x.050 to simplify my stock of chain, bars, and drive sprockets. I am completely happy with the conversion of the smaller saws.
Having said that, on the smaller saws, the .325 square chisel chain will cut slightly faster than square chisel on a 3/8 chain. That's based on actual time tests that several people have documented and posted on this site. But for all practical purposes, working in the woods, I doubt that you would ever notice the difference. The 3/8 chain is wider and takes a bigger bite, which means it could bog down a little easier on a small saw; particularly in larger wood. But I use mine as limbing saws, with 16" and 18" bars, hold them 'light' in the cut at WOT and they work like a jewel.