In reply to The Lemming: My eTrex still gives me perfectly good positional info. What additional features are you hoping to get? That'll make the biggest difference as to whether it's worth updating IMO.
I'd agree with mkean that a smartphone/tablet is going to be vastly worse in terms of battery life than a handheld GPS. The backlit, colour touchscreen is the main cause - that, and all the other capabilities and functions that you don't need/use out on the hill but which still draw power. My Android phone (which I have tuned reasonably well) will run for three or four hours with the GPS on permanently before needing to be plugged in to the mains for about an hour to recharge. My eTrex will run for at least twelve hours on two AAs, which can be replaced with a fresh set in less than a minute.
The Nexus 7 isn't anywhere near as ruggedly built, or as water resistant as an eTrex. There are a few ruggedised smartphones out there - my Defy+ is one of them - but I reckon it's still a lot less robust than my eTrex. The Nexus 7's shape is wrong for being stuffed into a rucksack, as well - it's quite vulnerable to bending. The monolithic shape and rubberised body shell of the eTrex means it's much happier being shoved in to any free corner in the 'sac.
The new Garmin eTrex handhelds support the GLONASS satellites as well as GPS, though I don't know whether that makes a noticeable difference in practice. The main other enhancement seems to be in the usability of the PC interface, so if you use that a lot then it might be one reason to give it a second look.