Topo vs. CityNav really depends on how you're going to use the device, particularly if you want to use it for things other than road cycling. For MTB and hiking then the Topo maps are more useful. OTOH, if you want to go road touring (also if you use the device for in-car navigation), esp. if covering parts of the country outside your local area, then the CityNav maps have useful POI data. You may not need to find the nearest Starbucks, but the location of the nearest grocery store can be quite helpful, as well as the location and phone number of the next motel when the weather starts looking worse. The CityNav maps cover all of the US and Canada for one price as opposed to the Topo maps which cover only individual regions.

If you can run MapSource on your Mac then I'd be inclined to go with the DVD - that way the maps are available for trip planning on your desktop in addition to being on the 705.