You can't have overlapping mapping for areas, which you seem to know. So the answer is you need to do a computer map recce of whatever route you plan on taking and see which mapping is superior for your needs. If the Topo USA has enough street detail in it for you, then it will probably be the better choice. Topo USA has a fair amount of tar mapping in towns as well, but City Navigator can't say the same about the mapping it provides for back country roads. Shouldn't take you long comparing them on your computer to decide which set of maps will be best for your intended ride.