i believe LTE-A is only available in internationala areas, like in Korea. not sure if its here in US though. AWS is available in some areas, specifically NYC. my friend has an S4 and after upgrading his radios, he is able to get AWS speeds. not sure if G2 will get new radios for AWS anytime soon though.
as for jumping from GNEX to G2, you will notice the huge difference. better in every way. i was waiting for the N5 but after reading the early specs that it was going to have 2300mah battery compared to G2's 3000mah, i jumped on the G2. even after N5 release, i would still stick with the G2. battery and screen (thin bezels) are the winners for me. i also kept my unlimited data while paying the subsidized price so i'm definitely happy.