The data protocols for various brands are different so whatever you do be sure everything you buy matches.

I found that some heart rate monitors were really vulnerable to transmissions from others in the area. My Garmin would mess up the Cateye on the bike next to me or the Polar on the gym equipment next to me.

I ended up buying a Garmin 310XT because it was only a few dollars more expensive and could be used everywhere. I've been satisfied. (One caution: I found Garmin customer service to be marginal at best; which is a real contrast to that for aviation and marine products. But once I got a device that worked it is bullet proof.)