I have both, as Techtool Pro since 12 or so finally supports SMART-Monitoring for external USB-Drives it upped it's game quite a bit. But if you use it ONLY for that DriveDX is the better choice, I guess. Some other feautres of TTP have mostly become pointless IMHO like emergency startup disk. The Cloning is still handy, though.
Volume Defrag and Rebuilt etc. is still great if you use HDDs. For SSD it's pointless and actually damaging.
But honestly I still can't decide which one to ditch. For now the Upgrade from TTP12 to 13 was just $24 a few days back, so no biggie.
BTW: you can get Drive DX right now at www.bundlehunt.com for $6