I tried the KA Costco model but decided on KA pro 600. (I used to have the Epicurean with the C-hook). The spiral is much, much better, I haven't found a need to knead on the counter top at all. I only turn the dough in on itself a couple of times after finishing the dough ball in the mixer. I've only had it a week now, and I've only made somewhere around 8 pizza's but they were higher hydration than I normally make.
I have not tried lower hydrations dough yet, and am a bit nervous to try them. I have read comments on the KA forum that the safety feature can kick on when trying to use any mixing level above 2, especially with larger batches or lower hydration doughs. This would mean that your mixer will shut-off and be unusable for 40 minutes. I believe this saftey feature is on all the new spiral mixer, but you may want to double check. I want to say that the s"pecial cost model" dose not, and has a higher wattage than the other 5 qts (475 wtt. I think) and I think it's the best deal for the value. I personally wouldn't recommend anything with lower wattage if your making lower hydration doughs with any regularity.
Now with that in mind. Since the spiral hook kneads more effiently, doughs require less mixing time and also may not need to mix at higher speeds.
Also keep in mind that the 6 qt mixers, aren't meant for smaller batches. I have found that I have to initially mix with the paddle, and finished with the dough hook, with or without the autolysis step.
Browse around the kitchaid forum, if you do decide to go with the KA. I'd wait until they are running their specials for a free attachment. You just missed the mother's day special.
I'll be trying a double batch of Randy's Thin-crust for the KA soon. I'll post as soon as I do.
Hope this helps.