.375CT outperforms .408CT, staying supersonic longer and has a wider variety of bullets to chose from. Try plugging the numbers into a ballistic calculator and seeing for yourself:
.375 cal 350gr high BC bullet @ 3150fps
.408 cal 419gr high BC bulelt @ 2850fps
At this point, the only reason to choose .408 over .375 is if you are using it for military applications and need the specialized AP rounds, in my opinion.