I dont know official technical reasons, but i've installed small watchguars up to an x500 and ive installed the sonicwall tz-170, the interface on the sonicwall is much earier to naviggate and understand, the configuration was definatley easier (the x500 took quite a few visits to get right) and the IPSEC vpn cluient for the sonicwall is much easier to get connected, it gets all the tunnelsettings etc for you on connection rather than having to pass a configuration file round.
I am now definately a sonicwall fan, and i think easier to work with than cisco pix too and i've installed about 30 501's
Skr