Actually, a better comparison would be ERP, or effective radiated power.

There are other factors that can dramatically effect range in addition to terrain and circumstance, those being frequency, height above ground, intermod, multipath, proximal channel ops, solar activity, antenna type/configuration, azimuth, and orientation.

For example, running a 5 watt portable at UHF business band into a magnetic mount 5/8-5/8 loaded coil antenna will give me about the same effective range as a 35 watt dash mounted mobile radio in the same band using a 1/4 wave antenna, given nominal variation in elevation within that range.