Couple of thoughts:

1) As michael stated, trying to create routes on the actual devices is hit and miss, just due to the small screen and general clunkiness of the software. Typical bike specific GPS units can be very good at navigating a route created on-line, just not as easy on the device itself.

2) What most folks tend to do is use one of the online route planning sites, such as RideWithGPS or Garmin Connect to do the route planning. You can create many multiple routes/courses to reside on the device to follow on specific days, so if touring as example and when you have WiFi, you create a munch of routes in advance.

3) To do off-road, I assume you want topo maps resident on the device that will show terrain features. Topo maps are not typically resident on a Garmin cycling computer, which mostly use a version of the OpenStreetMap road maps, or the Garmin Cycling map, nor are topo maps available for the Wahoo series of cycling computers. You can get free topo maps from OpenStreetMap, I can say I've had zero luck getting it installed on a Garmin Edge 1000, but I'm sometimes dense at instructions. Or you can purchase the region specific topo maps from Garmin, but they get pricey at $85 for the Northeast, as example.

4) You can create navigable routes for an off-road region, as long as the road/trail exists on the route planning software, but I've yet to see an off-road route give you turn-by-turn directions on gravel/dirt roads, as example, The bike GPS will show you a route you've planned on Garmin Connect, and will show you where you are, yet might not be able to say "in 3 miles, turn left on Nat'l Forest Road 431", as example. Certainly a cycling GPS that has topo maps installed will show you where you are, how far you've gone since you started recording the ride, avg. speed, time, etc....

5) Thus you then need to decide what you want this GPS unit to do. In some respects a hiking/outdoors GPS might serve you well, though you don't get the same cycling specific features of a device designed for cycling. One advantage to some outdoor oriented GPS units is they use AA or similar replaceable batteries, where as a cycling unit will need re-charging after 10-20 hrs. of use. You can use the external USB batteries to run a Garmin for extra power.