I've owned three different stand mixers ... well, the first one, I don't know if it really counts --- it was a hand mixer that had a stand so you could use it as a hand or stand mixer.
I've had both a KitchenAid 'Ultra Power' (4.5qt, tilt-head) and a 'Pro' (6qt, bowl-lift).
If it weren't for the increased capacity, I'd switch back to the Ultra Power (or maybe the Artisan, which is slightly larger). The main problem is the bowl-lift vs. the tilt-head. It's much less convenient to add ingredients or to get in there to scrape the bowl with the bowl-lift mechanism vs. the tilt-head.
The bowl-lift models also seem to be more temperamental about the bowl clipping in ... the tilt-head bowls have a threaded bowl, where you set the bowl down with the handle facing to one side, then you turn the bowl to lock it in place ... the bowl-lift models have two pegs that you set the bowl on, then tilt the bowl 'til it clips in. I've had a couple of incidents where I thought the bowl was attached right, but it wasn't. (I don't remember the exact situation .. it might've been that I had put the bowl away for storage, and hadn't clipped it in before using the next time)
Also of note is the 'pouring shield' that you can get. I think it comes automatically with the Pro series -- and you need it, because it's obnoxious trying to add ingredients in the first place. Unfortunately, the one it comes with is a single piece model, not the two-piece like for the tilt-head models. The one piece has a large gap in it so that you can slide it on and off ... which means that if you do something that's actually going to make a mess, it won't be contained ... and most likely, it'll be the mixer that gets sprayed and has to be cleaned up.
The only advantage for the 'Pro' is the size -- I can double most recipes without worrying about stuff trying to crawl out the bowl as it's mixing. But be warned that there's 3 sizes of 'Pro' -- the 450 (4.5qt), 500 (5qt) and 600 (6qt). I'd personally go for the Ultra Power over the Pro 450, and the Artisan over the Pro 500 if given the option. The Ultra Power has a more powerful motor than the Pro 450 ... that's not the case with the Artisan, unfortunately, but unless you're going to be running it for 30-40 min without resting at a time, you should be fine. (there's a thermal cut-off, so if it heats up too much, it'll shut down with no indication that it's done so ... but will work again once you let it cool off)