The 350B pipe was a modified 250 Team B pipe. Basically a 250 pipe with a larger stinger. The 350D pipe was a complete redesign. If I recall correctly the 350B was a bit more pipey. The D made more power over the range, but mostly down low. Keep in mind this was on Arlans 350PV ProX motors, and may statements can be taken with a grain of salt.
Jared, the pipe you saw was a 350G. I have the 350B on my spare bike.