Each unit has its advantages. The 310XT is a pretty chunky unit and the slowest to find a GPS signal, plus the strap pins are weak, but simple for running and cycling once locked on.
It really depends on what you want out of the unit. I've said many times before that the 705 is not a sat nav and those wanting one should be looking more towards the eTrex type units. As a young rider who is probably more interested in racing, the mapping feature is really an added extra in case you get lost on a long ride. The other features are more valuable to you.
I've only had the 500 a couple of weeks but I think it will become my favourite. It is built more like a standard bike computer (before Garmins I had the Sigma BC1606) and works just the same as the others with a clear screen capable of displaying 8 things. As I said before the twist lock mount is an improvement, and one extra display it has in the history is for maximum values....in fact for display of power data, 500 is the best. Only downside at the moment is the menu navigation which I have to really think about to avoid restarting or resetting the ride.
I compared the HR straps as well using the 500 and 705 on this evening's turbo session - no difference so that was probably just me. Interestingly the calorie count (which I don't record in my training log) was the only field where there was a difference. The 705 said 1050kcal while the 500 had 680kcal. Garmin have probably improved this in the new unit and it also takes athletic ability into account (eg: Elite/Good Amateur/Average Amateur/Slob etc...)