Haven't used the Garrett, but do have the DX-1 on an MXT and have extensive use of several other handhelds. As far as all out power and usefullness is concerned, the DX-1 beats the handhelds big time.
The only drawback is that it must be used with the detector it's designed for but the display works just like it does with the coil. As it is mounted on the detector, you can't forget it or leave it behind and it has considerably more power as it is using the detectors electronics.
The hand helds are nice in that you can use them with any detector, but can be left behind, forgotten or lost. Generally, most of the handhelds are underpowered although the Garrett, The Pistol Pro-Pointer and possibly one or two others have good depth.
If you have or plan to have several different models or brands of detectors, I'd go with the Garrett. If you're just going to use the DFX, bite the bullet and buy the DX-1, You'll love it.
HH
Bill