I'll let someone more technical answer in detail, but the basics as I understand them is small turbo can be modded to reach around 300bhp max, whereas a big turbo stage 3 should fetch around 330+bhp then you have the potential to reach even more if you want to spend your money on big mods.

IIRC small turbo has a more reliable gear box and less lag on the turbo thus feels a little more responsive, but big turbo is easier to modify to withstand much more power.

Basically if you're looking to reach more than 300bhp then opting for the big turbo seems the way to go, if 300bhp is around the figure you want to achieve then small turbo appears the better option.

Im sure someone will correct me if im wrong, they usually do lol