I have the Polar A5
. I would definitely recommend one with the strap the goes around your chest, I think they are more accurate than just the watches alone. I think most of the Polar models use a chest strap. My GF bought it for me since our gym was having a sale on them and at first I didn't think I would ever use it. After the first few times using it I was hooked and now wear it practically on every hike, bike ride, etc. I've found it to be quite motivating and it keeps me honest as well.