Help me decide. 310XT or Edge 500

I've been loving my garmin 405.  It's terrific.  I use it for runs, and I strap it onto my handlebars for rides.  Works fine.  However- with some long rides recently- I'm getting near the end of my battery life.  Today I finished and it was down to 3% battery.  It will be on the hairy edge of getting me through a 70.3 this summer.  When using the HRM, it only lasts 5:30.  It takes me longer than that to ride a century.  So, I thought it might be about time to get something with a longer battery life.

Garmin Edge 500:  Oooh.  i like all the fields it can display at once, and I wouldn't need to move it from my bike to my wrist at T2.  Downside- I'd be uploading 2 instruments, rather than one.  One more item to worry about.

Garmin 310xt:  A little pricier, but one unit will do it all.  quick release is easy to mount and remove from bike at T2.  Would take me through any future IM or Ultra run that I might be crazy enough to attempt.  Downside:  It's been around for a few years now- isn't it about time for Garmin to upgrade it? Also, a bit more expensive.  

So- I can't really go wrong with either decision.  But what am I missing here?  Tip the scales for me.  Tell me what you love about one or the other.  (yes, i know about DCRainmaker- Ray Maker's great reviews)