Another vote for IS350
I have test-driven both the 250 and 350 and ended up buying a used 2008 IS350 (It's one day old as of this posting). As others have stated, it's a big enough difference in power that you would notice it. Keep in mind that the ISX50 in general is a fairly heavy car, the extra power is very handy.
I think that if you go used, any IS350 below 2011 production year should be RWD as they recently came out with the AWD option.
2008 IS350 RWD // All Stock
"The early bird gets the worm, but the second mouse gets the cheese"