I have a dreadnaught (D-18GE, mahogany), an OM (OM-18V, mahogany) and a performing artist (GPCPA3, rosewood), so I can maybe help with the comparisons a bit. As far as playability goes, any of them play bluegrass just fine. As far as tone goes, they are all different, but I like them all (to me, the D18 and the OM18 have what I would consider a fairly traditional bluegrass tone). As far as volume goes, the D-18 is very loud, the OM is reasonably loud and GPC has less volume and it doesn't cut like some guitars. More volume is not necessarily better in all situations. If you play and sing by yourself, a flatpicked loud guitar can be too much. However, like Ol' Ben Kenobi said, if you are going to play with others acoustically, volume is a good thing.
The PA series has great electronics. They sound incredible plugged in, they are very easy and comfortable to play and I think they are a really great all around guitar (I've no plans to sell mine). With that said, if you are going to focus on acoustic grass with groups, in my opinion, the PA series may not be the best suited for that particular purpose due to the lack of acoustic volume.
The different body shapes and wood combinations all have their strengths and weaknesses. The OM and GPC are a more comfy body shape and size for me than the dreadnaught. The mahogany cuts better, but rosewood has a warmth that is pleasing and I think is more conducive to singing. In short, it comes down to what works for you and how you will use it. Don't get in a rush, and try to get to a store where you can play different shapes, brands and woods. If you can, find a great guitar shop with a quiet area to play a great variety of instruments. Near me, I like Charleys in Dallas and when I can get there I love Gruhn in Nashville. Ben goes to Artisan in Franklin, TN... I haven't been there, but the instruments on their website are incredible. Sooner or later, you'll find something that works for you. And by the way, you may not find "the" guitar with your first purchase. As time goes by my tastes in what I want in a guitar have changed, and I suspect it will continue to do so. Happy hunting!
Edit: As I was typing my long-winded thing I saw you decided on a Guitar. Good deal. I have played some 16 series that were quite nice.