for a few reasons.
First, for me, Louis is a family name in that, being French going way back, there's a lotta Louis up in there.
Second, Louis has a wonderful dual image of bookishness and cool-jazz working for it. I love that about it. It can be serious or offbeat, straight-edge or counter-culture, whatever the person likes.
Peter and Julian (if this is for you) is a gorgeous duo however. Very jewel-bright and vibrant, while Peter and Louis is more subdued and elegant. Tough call.