I have both 65S and 66SR and use them both on weekly basis for Geocaching. Both great devices. The 65 is a spare GPSr for me.
The Garmin Explorer software is CRAP but the GCLive download can be a geocache-lifesaver on the 66SR . The built in battery is an absolute winner, easily 30+ hours of geocaching-fun from one charge. I would definitely go for the 66SR if you have the $$$.
Just my 0.02.