stand mixer - reasons to go with a higher wattage vs lower wattage mixer
I was on a similar quest before Christmas. Here is what I learned: 

A  6qt will hold about 12 cups of flour, enough for 13 dozen cookies or 8 loaves of bread. A 5qt will hold about 9 cups of flour which would be about 9 dozen cookies or 4.5 loaves of bread according to Kitchenaid.That's good information for when you are working on large batches of something.

My own train of thought on power is: The more powerful machine would last longer than the smaller because it wouldn't be pushed as hard. This may or may not be true, I could not find any information on the average lifespan of these machines. My previous mixer was a 325 watt model (not KA) and was plenty strong enough for making big batches of cookies and kneading bread and is still in great working order. I wanted to upgrade to a fixed bowl type mixer.

It seems like most home bakers prefer the the 5qt. mixers. Based on reading numerous reviews at Amazon it appears as though professional/semi-professional cooks choose the the larger mixer for it's capacity and mixing strength, but that is overkill for everyday home use. The bigger mixers are heavier and taller (If you plan to put the mixer on your counter and there is an upper cabinet make sure to measure to determine that you have enough room). 
And there were a few reviewers that had both, liking the bigger model for big jobs &amp; the smaller model for small jobs. 

There is an excellent review of mixers at Cooks Illustrated.
Their first choice was a Kitchenaid Proline 7qt. Second place and best buy went to Kitchenaid Classic Plus 4.5qt. (275 watts which held is own in testing against the bigger/more powerful models) Third place went to the Kitchenaid Professional 600 series 6qt. All other models that they tested were rated below that.