Well I came to this forum leaning toward the Maxx HD and decided on the S3. Pulled the trigger today online via Radioshack - $99 for the upgrade and a $50 Google Play credit. Couldn't beat the deal.
What did it for me?
1) Onscreen buttons on the Maxx HD reduce the screen size while browsing. I opened the same website side by side in the store and the difference in the amount of the page you could view was bigger than I thought it would be. I would've preferred a capacitive rather than physical home button on the S3, but I'll deal with it.
2) I opted for replaceable battery over long battery life. Makes more sense for my situation. But this one depends on the individual.
3) Bigger selection of accessories. The HD is newer so I'm sure the options will grow.
4) 2GB RAM over the HDs 1GB RAM. Both are more than enough for now but 18 months from now that 1GB could start to drag the phone down.
5) GS3 will become a world phone with JB update. I planned on waiting for the update to buy but the deal was too sweet.
6) Camera - S3 wins big. Both in online reviews and in my own in-store tests.
7) Price - Haven't seen the HD below $200.
I'm sure I would've loved either phone, but this is why I pulled the trigger on the S3. If it doesn't wow me like I think it will I'll take it back within the 2 weeks.