Everyone has personal preferences and tend to like what they're used to. Your best bet is to use google and youtube and see some reviews of what's current. Then make your choice that way.
A free sequencer is Rosegarden. Some almost free softwares are Reaper and Anvil Studio. Both have trial versions.
Commercial softwares you might want to look at include Cakewalk Music Creator, Cakewalk Sonar, Abelton Live, Fruity Loops, Steinberg Cubase, Band in a Box, and even Garageband for the Mac.
One advantage is that the INS file for the XW-P1 has already been created for Cakewalk products. Other programs may not be able to fully exploit all the patches and sounds of the XW-P1 without a working config file. Then again if you look around the INS format has been around a long time and there are a lot of conversion utilities and softwares that can import the format.
Anyway, like I said, your best bet is to watch an hour or two of youtube and get an idea of what's out there and see which ones you like/can afford...
Gary