I have a mec and a ponsness/warren. I'd pick the p/w anyday. It just seems better made and it's really hard to make a bad shell on it. If you're going to make 3" or 3.5", the best bet is to use a single stage because I don't think any of the progressive presses do this. The p/w 375 does these shells and loads faster than the mec single stage imho.
I agree with much of what HammerMan says. I don't and have never used a Ponsness-Warren, but did sell components and parts to them when I worked as a Sporting Goods clerk in college.
I always thought of the PW's as the Cadillac's of the shotshell press world. As for MEC presses, they don't impress me. I learned to reload on a VersaMEC 700 press after 'grad-jee-a-ting' from the venerable Lee Loader.
Twas me I would use a DL-366 Hornady press for 'progressive' chores. I have always thought the Pacific / Hornady presses had a superior bushing philosophy/design - so good in fact one can interchange their various powder bushings or various shot bushings between Hornady, Ponsness-Warren, Spolar and RCBS! Folks i know who run the DL-366 oftimes have one dedicated to 12 gauge and one to 20 gauge.
Ballistic Products if you're not familiar with them, offersabout any component you might think of to make ANY 3 1/2inch 12 or 10 gauge round.