I'll just copy and past my response
from a thread that was posted a couple days ago asking the same question:
I have a S3 on the way. I'm going to try it out only because I'm addicting to having the latest and greatest. If I weren't that type of person, and I kept phones for the long haul, I'm pretty sure I'd hold onto the MAXX.
That said, here's what I see as the pros and cons if someone is thinking about switching:
Pros
- Larger screen with a higher resolution
- 2GB of RAM - With the way Android manages processes and apps, I'm not sure how big of a difference this will make now, but it definitely won't hinder anything.
- Much better camera - This is huge for me, but not for everyone.
- Removable battery - Nice for those times when you have a spare battery and no time/ability to plug into a charger.
- More accessories - All versions have exactly the same dimensions, so accessories for the S3 are universal.
- Some Touch Whiz features can be useful.
Cons
- Smaller battery (a huge con that could easily obliterate all of the pros)
- Worse reception - Samsung's CDMA/LTE smartphones have all had noticeably poorer reception compared to Motorola devices. While the reception on the S3 appears to be better than previous devices, it's still not up to Motorola's level, or even HTC's level. Definitely a major consideration if you are areas with weak signal often. It will impact not only your ability to send/receive calls and use mobile data, but also your battery life.
- Touch Whiz - This is really personal preference here, but I don't like Touch Whiz, plain and simple. There are some nice additions here and there, but overall, I strongly dislike the UI. The UI that Motorola uses, especially on ICS, is much nicer and far less intrusive. A custom launcher can remedy much of this, but not all of it.
- Inferior build quality - Even though they feel cheap, I think the S3 is built pretty well. It's just not the resilient tank that the MAXX is.
Overall, I honestly think the cons outweigh the pros for the vast majority of people. The battery life and reception of the MAXX alone are enough to wipe out any advantages the S3 has in my book. As I said before, I've got a S3 on the way only because I'm always wanting to try out new phones. But if I were the type of person who generally sticks with a phone for a period of time longer than 2-3 months, I would without question stay with the MAXX. If I do like the S3, and I find myself willing to live with the decrease in battery life and reception, I'll only be keeping it until the RAZR HD drops anyway...or the rumored HTC phone with a 1080p display.