I personally have a Cateye v2c so I'm pretty unbiased here.

I'd say go with the Garmin Edge 500. For not a whole lot more money than you can buy the v3 for, you get all of the features of the v3 and compatibility with ant+ devices (such as power meters). Oh, and you get gps mapping including terrain, elevation, temperature, and a bunch of other cool online features.

The only reason I didn't go this route (and got the v2c instead) is simply because at this stage of my training, I would like to gauge my heart rate/stress level manually rather than having computer data. In the future there's a good chance I'll get the Garmin.

As a side note: It would have been rather intelligent of me to just get the Garmin in the first place and just not use the HR monitor for a while... but that's not the case. Mistake made, noted for the future.