I bought the Vista HCx. Price was a big one - I didn't want to spend more than $250 for the unit (by the way, check Amazon - can get one for around $220 or so). I also wanted one that I could take if I go for a hike or in a car and one rugged enough that if I dropped it a few times I wouldn't have to worry. But mostly I wanted a GPS for navigation. While the heart monitor and cadence sensors are cool I can use a second monitor for that and still have enough money left over for the maps (don't get the Topo - it sucks dead monkey gonads).
I would never buy a unit that has an internal rechargeable battery. First off that will degrade and then it's useless unless you hack it or buy a new battery for something exorbitant (usually). As you say, the AAs are easy to carry extras around (or if you want to be really slick you could do something like
this with rechargeables). Even with my Vista's more sensitive GPS receiver it goes wonky from time to time, I couldn't imagine what it would be like with a lesser one.
I put up a thread with a bunch of links in it for the eTrex
here if that's useful.
And the altimeter is fairly useless unless you want to know if a storm is coming.