Another thing to consider if you are just looking for a base model HRM is strap comfort. I loved my Polar (never felt the strap at all) but I know a lot of people find it uncomfortable. My Garmin strap sucks - it hurts after awhile. I also have a Timex Ironman HRM and that strap is ok but not great.

If you will ever wear your HRM for swimming, then go with Polar. They make the only ones designed to be used while in water. Timex is waterproof to prevent damage, but it won't transmit under water. The problem with the waterproof Polar is that you cannot change the battery yourself so when it runs out, you need to send it in for service. Timex (and I think Garmin) both allow you to change the battery in the strap yourself.