As far as I know the 346XP was delivered in the US with .325" x .050" chain. The bar should be marked with the appropriate chain size. Mine has been changed to .375 x .050.
.325 or .375 refers to link length, and it must match th drive sprocket and bar end sprocket. The .050 (or .043, .058, .063) refers to the width of the drivers, and need to match the groove in the bar.
I don't think that there will be a big difference in cutting speed with either .325 or .375(assuming the 50cc 346XP), but if you're cutting hardwood, round chisel or semi chisel should require less sharpening.