I currently have a Garmin 405, with a bike sensor.
It used to have a crap battery life (less than 3hrs), but a few months ago it went back to Garmin for repair cos it wouldn't charge up, and the refurbed one they sent back as a replacement has a excellent battery life, I reckon at least 7 or 8 hours - which is plenty for my training.
I really like the sound of the 910xt, and want one, just cos!!, but can't justify it on cost when what I have works absolutely fine. The only bit I'm missing out on is the swimming, and from what you say, it's not great anyway, especially for crap swimmers like me. That makes me feel better 
What does it give out of the water that you don't get on the Garmin 305/405 to make it that much better??