I'm definitely buying a S5. I'm tending more toward SS or Titanium, as the crystal on my aluminum S2 has been scratched up almost since I owned it - I want the sapphire crystal for durability, not as an investment.
I'll keep my S2 as a while I am sleeping watch, to continue to use AutoSleep, now starting the day with a fully-charged S5.
I don't understand people who say that AOD isn't that important. I swear daily when I look at my watch and see nothing but a black screen, particularly when I am out on a run. Sometimes even the exaggerated wrist raise don't light my watch (or its is delayed) - I have to tap it all of the time, and there have been times that I have tapped just as the watch lights up from the wrist raise, and the watch mistakes the tap for an activation of whichever complication was hit. Always on display for the win for me. I would have still bought an S5 without it, but I'm really happy that the feature is there.