Well, the trouble with showroom viewing is that with any decent TV manufacturer displaying sets in a store, you're not getting 1) in many cases, a viewing environment like your home; 2) more importantly, a picture anywhere as good as you can get if you get the set calibrated by someone who knows what he or she is doing. Some TVs are just ok out of the box. Some are horrible. And almost all of them come with their settings torched, to catch the eye. You can help this problem a little by getting into the user menus, but there is still a LOT you can't do that a calibrationist can.
So, differences in quality seen in stores are very, very difficult to transfer to what your experience would be in your home.
Nonetheless, I thought the Mits was a little better in HD than the Pioneer--smoother and truer colors, even after user menu adjustments. Both were bad on SD, but what would you expect?. I thought the Mits had slightly better shadow detail on 480p.
If you don't mind, could you pm me with the price you're getting quoted on the Pioneer? Is it the 730HDI? If so, Costco must be blowing those out seriously to come in $500 under the Mits 413. If it's not, you might be comparing an '03 Pioneer with an '04 Mits (which is fine, you just need to be aware of it).