It is hard for anyone to make that decision for you but this is what I would think about before making a decision.
SD Card Pros:
Quick to backup (1,2,4 Gig)
Can run multiple cards (ex 1 or 2 for maps, another 1 for testing 3rd party apps)
SD Card Cons:
Not a lot of storage for additional info (MP3, POI/waypoints, extra Maps etc)
Hard Drive Pros:
Lots more storage than any SD card can currently offer
Can store any file on it and use as a mass storage device
Hard Drive Cons:
More susceptible to vibrations (ex falling of windshield mount onto dash)
Could theoretically use mare battery power (as it has to spin the drive up and down)
The 910 comes with lots more accessories out of the box than the 510.
Remote, US & Euro Maps, Mic for Handsfree calling (Bluetooth linked to cellphone)
Also has text-to-speech
Tomtom runs a version of Linux for an OS so the possibility of 3rd party applications down the road is good, although it could also lead us to looking for that magic reset button and paper clip more often.
I have not played with the Garmin 2720 so I am unable to comment on it.
