I use the 610, I got it before the others were available. However, I find that using either the Forerunner GPS system, or the Fitsense footpod system is invaluable to my running. I like the freedom of not sticking to preset runs, and traveling yet still knowing what my mileage is. It is an additional measure of how I am doing - I run by heart rate, but sometimes my pace is way off at that heart rate, even though my perceived exertion is not.
I have 2 systems, because neither is without problems. The GPS drops me sometimes. The footpod - an accelerometer, which is what polar uses, I think, has to be calibrated. Every now and then I'll carry both to calibrate the footpod. I race with the footpod, not the GPS, because my pace matters more to me then.
I've been considering the same options that you have, simply because it would be great to have both pace and heart rate on the same graph. If my pace and heart rate drop, I'm sloughing off. If pace drops, and hr goes up, I'm probably going uphill and its appropriate. I love to analyze stuff, though.
I'd love to have the cycling features. I can't justify the cost at present. I'll probably wait until my current system dies before changing, I'll just keep running around with 2, and sometimes 3 watches on in the meantime.
One difference between the 2 I noticed was that they come with different chest straps. One is more flexible, and presumably more comfortable, but is not as water resistant for the swim. If you get that chest strap, I'd like your opinion on it. Mine will need replacing at some point soon.