You have to make the decision yourself.
But you should analyse what you gain vs. what you lost in switching to S3, and give a 0-10 points weighting/importance to each point according to your needs, with 0 being useless to you, and 10 being most useful. Think hard, some gains/losses are not obvious.
For example (and example thinking behind the weighting):
- (+3) Better CPU/RAM/Storage (The PLAY does okay with the tasks I throw at it, but better hardware would still give better experience.)
- (+4) Friends will no longer say your phone is thick
- (+7) Better Camera capabilities (Oh! I have kids and a good Camera in a pinch is really useful for me.)
- (+1) New Android 4.0 Features that the PLAY does not already have
- (+2) Larger Screen
- (+4) It's currently in promotion which nets me $40 savings
- (+2) I'll end up with 2 android devices (which I think can be somewhat useful in some situations)
- (-7) lose built-in gamepad, Xperia Play Optimised android games, and emulators. (I think not many future Android games will support Xperia PLAY, but I'm still enjoying current ones that does, and emulators a lot! Best alternative on S3 is clip-on gamepad but it's another piece to charge; another piece to bring with me and may not have so much game support.)
- (-6) Opportunity cost of 24 Months Contract (I'm on the fence about the S3... I probably won't bother changing handsets again for the duration if I commit. By keeping the Xperia PLAY, I can still wait until I'm certain about the S3, or another handset.)
- (-4) all those accessories for the PLAY cannot be used with S3 anymore
- (-2) Financial Costs of device + accessories you plan to buy $800 (pffft..., I'm rich.)
- (-2) Environmental impact (I feel bad about changing handset while it still serves all my purposes nicely... well, a little anyway...)
Gains = +23 points
Lost = -21 points
Total = +2 points
In this example, a result of +2 points, out of the scale of +23 points, is not a very convincing win for S3. If you want to be rational in this decision, I would suggest pulling the trigger only if it's a little more convincing or if there's some gains you want to put a +10 into.