The point of a 'Google Edition' phone is that it receives the 'Nexus Experience.' That means it is pure AOSP. It will get updates whenever Google releases them. A Nexus 4, Galaxy Nexus or Nexus 7 won't get updates faster than a Nexus S4.
That could be very true and would be great. My point is though that all the features that comes with the S4 wont be on it and you wont even get the added camera features because those are all Samsung added things.
Dont know about you but though i dont care for the TW launcher,(i use Nova or Apex) i do like other aspects of TW and i would definitely want all the camera features. You will get Google Wallet though which is something that doesnt work w/ TMobile because they support the Isis app instead.