I use the GPS Routes primarily (assuming the route not using bike paths, divided highways, etc. which throws off the GPS) but always verify with the maps. I also familiarize myself with the maps that morning so I sort of know what to expect, i.e. I make a turn in Town X then go 18 miles before turning onto Road Y.
I prefer Routes over tracks as, at least with my device (64s I think), I get a beep to notify me of an upcoming turn whereas the Tracks are silent. This is my personal preference. Others have the opposite preference. Tracks are only my preference when using a lot of bike paths since my device's routing can not handle paths.
That said, as ACA says, do not rely ONLY on the GPS Routes. While not certain, I think some of the newer routes do not offer GPS Routes, only GPS Tracks. For instance, on the PPP route, I have had to create Routes based on using the Tracks as no Route waypoints are included, only Service waypoints.
Speaking of which, I have found the Services data to earn a grade C at best. They are frequently off by a block or more, missing key info, hard to search on outside the immediate area, and are not kept up to date. Why they don't keep the data up to date and give more info than the maps can, I do not understand.
In summary, both are good provided you know the pluses and minuses of each format and use them accordingly.
Hope you have a great ride, John