Well, I got about an hour and a half worth of fieldwork in with all three of them and here's what I think:
Plastic: TP sexy and awesome as all-get-out. VIP - not my favorite run. Could be because it was freezing, but my discs were very hard and stiff. I prefer grippy and flexy.
Stag: 173 VIP I threw this guy next to my domey recycled saint and my star teebird for comparison. I was throwing into a headwind, tailwind and the occasional calm. All said and done I have to agree with what ManU has said before. Very long straight flight similar to a saint or teebird, but the fade on the end is very had/abrupt. The disc it most reminds me of is a sword in fairway version. I like it, but I'm not sure what to do with it because my teebird and saint are straighter. Not to open up a giant can of worms, but I would not say it is more stable than a saint or teebird (maybe a tad), just that the fade at the end is different. It will turn a bit if you crank it, and it definitely did NOT fight wind well. It went about 420 for me, same distance as my saint/teebird. Make no mistake though: the glide is insane and this disc definitely bombs without a great deal of effort.
Underworld: Loved it. Threw a heavy VIP and ran it through the all it's paces. It's basically a westside river or glidier leo in flight. It was more stable than I thought it would be though. Thrown with a touch of hyzer it flip up to a frozen rope flight. Give it some height and a pretty flat throw with good snap and it will BOMB. It out drove everything I threw on a few occasions. Thrown flat it will hold it's turn almost to the ground and fade back a touch at the end. I also cranked it a few times into some big sky rollers. I think if you don't have a rocket arm you will love this disc and I think you'll get some very easy D out of it. Even if you have a big arm it's still a very, very good understable fairway.
Tursas: 178g TP. It flies exactly like a mini underworld. No other way to say it, but the flights were nearly identical just that the tursas was shorter. Hyzer flips to straight. Does big anhyzer bombs well. I found it can handle more power than the fuse but was still a bit touchy. I need more work with this one...and more money to buy more underworlds
I'd be a much better disc golfer if it wasn't for all the trees
I want your AMOEBA STINGRAYS! (esp. glow)