Well the Magic is mostly the exact same hardware as the Hero. All you get extra with the Hero is the 3.5mm audio jack, and the 5MP camera vs 3.2MP of the Magic, and the white Hero has that teflon coating. Other that, they have the exact same hardware.
The only other benefit of the Hero is the SenseUI, which if you really wanted, you could root your Magic and install a modded Hero ROM.
If you're on T-Mobile USA, the Hero will lack the 3G compatibility, so it would be a bit of a compromise without much gain.