This comming from an Atrix HD owner.
There is basicly no Dev community except for me and two others.
If you want a phone you can flash, root and hack then stick with Sony, HTC One X, or SGS3.
Honestly the HD bootloader will never be unlocked. I came from an Atrix 2 and they still have yet to see ICS.
They have root, but its useless cause we cant flash firmwares or kernels.
The Atrix HD has no root. Motorola spent the time to eliminate EVERY exploit. Signature checks are so anal that if you tryed to flash over a root with recovery, you'd at best stick your phone in a bootloop and the battery cannot be removed.
The only good thing is that the phone is fast and it has ICS and it's pretty stock looking.
Sony has way more features, a better dev community.
At the cost of the s3 (even though its as good as the s4, the s4 will outperform with future developments and has less battery draw)
Honestly, stick with the Sony.
at first I hated the Atrix HD, but its teaching me a lot about the Android operating system. I can root just about any ICS phone unless its a Motorola Atrix HD
another thing to hate about the Atrix HD was that they removed Webtop OS so the phone can no longer be used with any of the Lapdocks. Which sucked for me since I had one. Sold it to a friend for $60 bucks.
Sony. Stick with it.