The XPs (Palomino core) have SSE optimizations (like the p3) and also have data prefetch. They also run cooler than the Athlon-C 1.4's. The XP's are also multiplier locked with laser cut L1 pits that make it a lot harder to unlock your CPU. The XP1600 beats the 1.4 by a few % in most benchies. Check the AT article on the first XPs for actual bench numbers. I think the general rule of thumb I've consistently seen is if you are running a DDR board, go with an XP, if you are running a PC133 or PC100 board and already have a 1.4 T-bird then there isn't as much incentive to upgrade to an XP.
Chiz