Need Heart rate monitor/cyclocomputer choice advice

[OakLeaf]

If it's important to you to "target events when I experience symptoms", then grade of hill is something to consider. It's the nature of GPS to be quite inaccurate when it comes to elevation, and none of the Garmin wrist-mounted units offer barometric altimeters; still, the GPS data would let you note which hills were a problem for you, and compare them against other hills in your area, etc.

I believe that the 310xt cannot give HR alerts unless you program a workout (which can be a very simple workout that says nothing but "maintain HR between X bpm and Y bpm). I'm not sure about this, since I use HR during the workout only when it's a multi-step workout I've programmed, but I seem to recall complaints about this on the Garmin forums.

Just FYI, the 705 can navigate you in your car as well as on the bici. It's not ideal - the screen is much smaller than motor vehicle units, and works in portrait mode rather than the landscape mode that most motor vehicle GPS use - but that's something to consider in terms of value for your money, since you mentioned the value of having GPS without navigation.