I am thinking about getting one so please tell me what you like and don't like about your galaxy s3.
Only thing I don't like is the inability to move apps to the SD card because of the 16GB internal memory. Everything else about it I absolutely love. I had a problem earlier this year receiving phone calls that was an AT&T issue but mistakenly the kiosk mgr. at my local Costco thought it was a problem exclusive to S3's so they exchanged mine for an LG Optimus which I absolutely hated. When I found out the next day the issue was not exclusive to the S3 but a random issue on all AT&T 4G phones I got another S3. I highly recommend the S3 and from what I read another poster say on another thread the S4 is even better. So if my upgrade were available now I'd get a S4. I also had the original Galaxy (AT&T Captivate) and liked that phone very well.
Then get the S3, I'm sure you won't be disappointed. BTW, I would not pay any more than $30 to upgrade phones and I think the S4 is still $200 on AT&T. Reason I wouldn't pay additional for the S4 is although it does have improvements it's not a substantially different phone.