PHENOM II X2 550=3.1Ghz and PHENOM II X3 710=2.6Ghz.....how about this difference in Ghz..
no problem you can just oc the x3
still x3 win?
and yes if the app is multi threaded then the x3 will win
you should go with the x3 if you want to be more future
however, as someguy7, said you can also go with the Athlon II x4
it will do slightly better than the x3 in some apps but if you want gaming then the x3 is best