I have a MEC grabber in 410, and while I can get about 400 an hour if I really hump on it (I would expect 12 gauge to be faster) However I don't think their is any comparison in the ease of use, and general flow of a Ponsess Warren Compared to a MEC.
Even in single stages, a PW 375 is much faster production, and a much smoother flow then a comparable MEC.
If you want to make excellent quality ammo, in a reasonable amount of time at low cost, then MEC's will do the job. If you want excellent quality ammo, in a shorter amount of time, and want to enjoy your reloading time, I suspect the other options are more up your ally.
I can't 100% attest to a PW progressive yet, but I should get to in the next week or so.
I like the fact that the Dillon has shot and powder that are hull activated, so you shouldn't have as much trouble if you miss a hull or anything like that. I have never seen a Dillon shotshell press in person, but I would seriously consider giving them a try if they would do 10 gauge or 410, however in those gauges your options are much more limited.
If you want a lot of opinions on various reloaders, shotgunworld's reloading forum is a good place.
I have not had an opportunity to use too many presses yet for shotguns, but in my opinion the PW stuff is well worth the added cost over the MEC, but the MEC will turn out ammunition that is every bit as good.