I would go with either a 500 or 510. This will at least give you room to grow...if your decide you want to use a HRM or a cadence sensor in the future.

The other benefit of the 510 is that it picks up the satellite signal faster than the other models.

Between me and my GF...we have the 200, 500, 510, 910, and the Tactix...I like the 510 the best.