I was told by an AT&T store employee that the battery life on the S6 is not that great and the glass is different as well (not as strong/scratch proof). You also can’t change the battery out if you start to have problems (can’t remove back cover). Add to that the fact that you can’t upgrade the memory. I maxed out my 16gb S4 and was about half way through a 64gb memory card as well.
As for the proprietary plug on the S5, I believe that is only needed for the fast charge option. I was told you can still use a standard charger with the phone - it plugs into just one side of the port.
I bought Woot S5 couple months back. Was a Verizon version and just a few scratches on screen, but alot of damage around the edges. Deal killer was a bad charging port. Woot offered me $20 to keep “as is”. What a joke, I returned for full refund. Am I tempted to try again? NOT!
The S4 is a great phone but the S5 does have a few upgrades. The processor is faster, it can hold up to a 128gb micro-sd memory card, the rear camera is 16megapixel (vs 13 for the S4), the display is slightly larger (5.1 vs 5). It’s also supposed to be more waterproof and dust resistent and it has a fingerprint scanner and heartrate monitor if those are important to you.
I got the S5 when it was $199 and am happy with it. It’s quite worn around the edges, but the screen is fine; it obviously had a protector. Not sure about battery life, but it’seems good for a day at least. I’m using it with Consumer Cellular / AT&T.