I always use both at the same time. When using routes, the gpsmap tends to recalculate and you end up with a different route than the one you wanted to ride. The route provides you with turn-by-turn directions and estimations of time and distance, while the track makes sure you don't stray away from the route you planned to ride (when in doubt: follow the track, the gps recalculates the route and usually your route is back on track
).
Also, a route is easier to make on the computer, because you just have to click on a few points and the computer does the rest for you. I always make routes on the computer and after that convert them to tracks with a few mouseclicks, using a little piece of software called wingdb3. I just load up both route and track on my unit et voila, good to go.