Right now my G3's are in for repairs after 5 good years of use. So I'm using my Patagonia Rio Gallegos Zip fronts which I bought as backup.
Some observations:
Both my G3's and Patty's are very well made and seem to be durable enough to withstand years of abuse whether crawling over boulders or whacking through blackberry bushes etc.. If you plan on fishing in those type of conditions, a high end wader with many layers is essential.
My G3's are not zippered but the Patty's are. Which do I prefer? While zippered is a nice convenience to have, its up to you whether you think its worth the extra coin. I kind of like the storage that non zippered waders offer though.
As for the fit, I am 5'8 and about 165. I use a MK in the G3's and an M in the Pattys. The G3's are only very slightly wider in the chest. The Patty's fall in between Simm's M and MK. Overall though I prefer the fit of the G3's over the pattys. The pattys don't have unbuckling suspenders which to me is a bit of a pain. Regardless of what you do, if you are spending this kind of money on waders make sure they fit properly. The majority of premature failures from waders stem from poor fit. I have waders for both the summer and fall/winter months. Wear what you would wear underneath when fishing when you go and try them out. BTW, the Rio Gallegos waders have knee pads which can be removed from inside your waders. Awesome when you are kneeling down on a rock or the ground.
Breathability. I was a bit worried about whether the Patty's with their H2No fabric would offer comparable breathability to Gortex but I am glad to say that it works well. The Patty's have a lining in the neoprene feet that works great when you are in the water but a little warm and clammy when you are not. Overall though I am just as comfortable in my Patty's as in my G3's.
As for Warranty and service, much has been discussed here about the inconsistency of service from Simms. I've always had good service from Simms whether its for their wading jackets or waders. It seems Simms and Patagonia have a difference in philosophy with their High End products. Simms makes and services their Gortex waders in Montana. Their strategy is to treat high end waders like cars. Spend a good bit of coin on them initially and pay a little every so often to maintain them. They expect your waders to last a long time. Patagonia would be more apt to replace rather than repair. Their waders are made overseas and it would be cheaper to just send a replacement than to send them away to try and fix them. Either way, always have a backup pair to use because it can take a month or more for your waders to get replaced or repaired.
I am happy with both products. I don't think you can go wrong with either choice.