The main difference between P45 and X48 is x16 PCIe. X48 was considered better at gaming & video benching because of the dual x16 performance if you ran crossfirex. Running 2 video cards on a P45 wasn't as good for benching. Real world though, you couldn't see the slowdown because it was fairly small.
The P45 is the better at overclocking 45nm quad core CPUs, but the X48 was really good with 65 or 45nm dual cores. If you want to run CPU benches on quads & single card video benches, look for P45. If you are interested in crossfire video benching, you need to get an X48.