I have the poalr 510 which I notice no-one mentioned. It's quite a bit cheaper than the 710 which has altimeter and watts as the only difference that I can tell. It has cadence also and can be uploaded to the computer. BUT the watch is HUGE. Of course you can always mount it to the bike. I am sure the 710 and 720 are the same. The difference between 10's and 20's is the watch case. 10's are rubber or plastic or whatever. 20's are stainless.
Anyone considering a polar should go to:
polarusa.com
Select cycling (or tri) and then choose comparison. I believe the 625 is the most expensive and I can't see that it adds much.
Finally, I got mine on ebay. Brand new in box. Waaaaaay cheaper than retail. Ebay always has tons of heart monitors for sale.
Happy hunting.