Its slightly easier to find mods for the Series 2 3800. Alot of folks around here swear by vendors like Intense Racing and ZZ Performance.
I wouldn't let that be your sole determining factor though. I have a 1995 SSEi that I recently purchased. I felt it was the perfect car for me the moment I sat in it months before I finally did buy it. I've been in a 97 SSEi and I prefer my Series 1. It has alot of cajones
By the way, I believe the Series 1 L67 has a bit less horsepower then the Series 2, but less built in torque management. Others will chime in with the specifics.