I have similar thoughts as others. If you expect to really use the suit, get the Kokatat for the warranty and customer service. The extra $$ will be worth it over time.
If you're tight on cash, get the Amp, but don't expect to stay as dry and the warranty and customer service are not at the same level as Kokatat.
I needed a suit and could not afford the Kokatat. I got the Amp in March 2016 and price was my deciding factor when I got it. I'm 5'10" and 200lb. I got the XL and it was WAY to big. I exchanged it for a large and that fits me fine. The Amp is my first dry suit so I can't compare it to Kokatat on performance, but here is what I like about it.
It fits my stocky body type well and it is easy to get in and out of. I've used it in 80F weather on 40F water, with lycra top, longjohns, and swim trunks underneath; The suit was comfy. It breathed well. I was only a little damp at the end of the day. I've used it in surf in the Pacific with 52F water and 55F air a Morro Bay with full lycra and polyester longjohns underneath. The soaking from the surf seemed to reduce the moisture transfer a bit, but I was still comfortable. The zipper pocket on the left upper arm is nice to have; I keep my rescue inhaler there. The tunnel works well - snugs up easily. The neck gasket fit me well with no trimming. Never had leaks in the gasket areas while playing in surf or practicing rescues for a few hours in flat water.
Gripes: The socks get pretty wet inside, but that may be due to small leaks or sweaty feet. I can't tell. It has not mattered enough to me to figure out exactly what the issue is. Even with a little lube, the zipper is stubborn coming over the shoulder and easily snags fabric underneath. I have a few holes in my favorite tops now (that's probably just me being my usually dorky self). The material gets fairly rigid in air temps below 50F. This also may be a common experience.
Concerns with the Amp: Gaskets discolor quickly with any type of sunscreen. I talked to a Stohlquist rep about it at Lumpy Waters in 2016 and he told me the discoloration was was expected and to rinse it well and avoid contact with chemical sunscreen; if the discoloration persisted, send it in for repair. My gaskets are fine after a good rinsing.
I can't say if Kokatat has the same issues with the gaskets or socks.