I thought I'd weigh in on this thread. I have been using prodigy mids for years, and recently branched into the putters and fairway drivers. I think Prodigy plastics are awesome. I use mostly Prodigy discs when I play.
I've been throwing some 750 plastic lately, and agree it's stiffer and extremely durable. I have noticed, too, the OS qualities some have mentioned. But I think the 750 OS qualities are not because it's more OS than 400 or 400g, but because it's so tough that breaking in takes a LONG time. I thought the 400 took a long time- and lasts forever- but the 750 takes even longer to break in.
I do feel like the 750 releases the best of the plastics, and *slightly* better grip when wet than 400 S and G. Is it worth the extra cost? I don't know yet (and I only buy the x-outs to save a few bucks) I have a number of 400 and 400g discs that have yet to beat in to US or flippy. The 750 discs I have are still relatively new, so it will be a year or more before I can say. I prefer the release of 750, but in the winter, it's very stiff. 400(S) is the more flexible of the 'premium' Prodigy plastics, and I think it's better for cold weather play.
I love the 300/350 for wet play and putting. Great grip, and to me the 300/350 just feels better than most comparable- dx, classic, dpro, etc. I have a few putters and mids in 300. the 350 is firmer and great for more glide, distance and carry.
that's my $0.02