Is there a substantial difference between the 2 sizes and will the 7 qt work just as well for smaller batches of dough or icings?  I found a used 7 qt commercial KA on craigslist for $200.  Didn't originally need a 7 qt, but the price seems right.

go for it.  It is slightly different as the bowl is slightly wider and you get that super duper wire whip with it. or don't and put the money in your 401K.

I own a 7 qt Rival, and it mixes small batches with no problem. Good luck with whatever mixer you purchase.