Right now for me, it's a tie between the S6 and the G4
they are both great phones with very good cameras.
if i were going to buy one TODAY, it would have to be the G4 because of the replaceable battery and SD card slot. both of which are now missing from the S6.
that being said, i would wait a couple of weeks and be looking at the 2015 version of the Nexus 5 and the One+2. those are the 2 on my radar to replace my Note 3. if i even decide to upgrade. i still love this phone.
the T-Mobile plan itself is pretty cool, if you use it to it's fullest. i would much rather jump on their new 4 line for $120 deal. EACH line gets 10GB of data. that looks amazing right now.