The Artisan is slightly larger (5 qt versus 4.5 qt) and more powerful. You can spend an extra $14 and get the white or black Artisan - I'd suggest doing so. You never know how much power you'll want in the future and the price difference isn't that large.
I have an Ultra Power (not a Classic). The saleswoman explained to me that the difference between the UP/Classic and the Artisan is that the Artisan does not have a heavy duty transmission. I love my UP but am considering upgrading to a 600 series...mostly due to my new found love of baking my mixer has brought into my life . :) Good Luck!!
I can tell you that the Classic spins noticeably faster than the Artisan when not under a load, since I have had the Classic for 15 years now, and just got an Artisan to take to Florida where we now "Winter". I spun them at the same time and the Classic spun faster at all speeds including the highest. They weren't under any load, but to me this means that when whipping cream or egg whites, they will whip up faster with the Classic than with the Artisan since they present basically no load to the mixer.
the only difference between them is the bowl on the 150 or Artisan is alittle bit bigger, that means the beater is also alittle bit longer. The dough hook and wire whip will work on both. The mixer them selves are the same.