This post is from Ron Baldonado and I think it's valid for the 70 series also:
Garmin 60 series VS 64 series hand held..
60 series has many limitations
Only accepts 8 gig mini SD card.
Only will load one map per card, no layering of maps..
You need to change SD cards to change maps, if you load a new map it will overwrite the existing map.
60 series cannot accept map over 2 gig.
But it does have a serial charge port.
It will charge NICAD batteries on a mini USB or serial port charge.
64 series
Will support many maps and map layering.
It will take up to a 128 gig card if SD card is pre-formatted as a 32 bit partition.
64 will accept maps over 2 gig.
Screen size is the same.
64 does not have a serial port.
64 will not charge a NICAD batteries unless the battery pack is bound together as to push the charging button in the battery case, single AA will not activate the charging abilities of the unit.
Physical size and screen are the same.
Convenience and ease of use the 64 is a better bet.
The 60 is just as good of unit, however, it takes more effort to perform the same tasks.
Ride with the wind and keep the rubber side down, the Webmaster.