Garim 920XT or 735XT

The 735 should still work with your HR strap. DC Rainmaker has a nice overview here:

http://www.dcrainmaker.com/2016/05/hands-on-with-the-new-garmin-fr7...

Thanks - that was very helpful

When any of the optical-HRM Garmin watches are used with a chest strap, it will combine both sets of data to get the best HR graph. So yes you can use your existing strap- but you don't need to.

Thanks - good to know.

Be aware that the 735 has a battery that lasts 14hrs in GPS mode. I think its 14 hours. That may be the reason it is smaller than the 920. Smaller battery. If your going to do full Ironman distance, well, the 735 would be long dead before I finished.

Also keep in mind that optical HR isn't as good as a chest strap. It good but not as accurate and takes a while to register changes. That sort of turns it off for me since why pay more for less reliable data and which drains the battery more? I went with the Fenix 3 which is basically a 920 but looks nicer. It costs more though but I found a decent deal on Ebay to knock the price down to the 920 price. If I had to chose between the 920 and 735, I'd go with the 920. I don't think I'd need over 14 hours of battery life but who knows. Battery life also get worst with age so that's always something to consider as well.