Belden speaker wire
Check the belden website for more specs but here are the basics.
5100UE - less quantity, larger copper strands (15-20).
5100UP - more smaller copper stands (50-70).
The higher strand count of the 5100UP makes it slightly more flexible. Both are the same gauge, same insulation, and both make great inwall speaker wire. There is no problem with different wire for one speaker.