Ah, Dave brings up point and another difference.
Are you going on a single or multi day trip, be it adventure or road, you, a few friends with GPS units like yours ?
A route you built is the way to go for reasons given above. Its more mental work involved pre trip, but way less once executed.
Going to or hosting a DS event like Dave mentions and the masses are involved ?
A track is your only option.
But again, we are talking about 2 different animals here. Ill say that most guys use tracks, its just easier to share the intended way or route, between guys. Just about any good GPS will let you follow a track-line and it doesn't matter what version map-set it has on it. Meaning, lets say you have a 2009 map-set and your friend has a 2012 map-set. If you build a route, like I advocate, on your 2009 map-set in Mapsource for example, and email your friend that route, his GPS with the 2012 map-set will puke it out. Error: "MAPS DO NOT MATCH" "WANT TO RECALCULATE" ?
Of course if he pushes yes, the unit will destroy the route. Ill explain what is needed to remedy this later.
You don't have to worry about any of this with a shared track. The GPS will just ingest it. However, thats all it will do. You are left to staring at the screen to make sure "your on track" .
I need reading glasses, so for me on a huge, 2,500 mile, 14 day trip, this just takes my attention away from the landscape a little too much. And if I have a plane ticket waiting on me at the end of the trip, say like this years trip, in Reno, I can casually, glance at the ETA through out the trip to get a idea of how we are progressing. I also get "Distance to next turn". Huge. Example and there are many of them, TAT, Oklahoma, we make a turn onto this slick dirt road through the prairie grass, "Distance to next turn" 45 miles". Nice, I can pilot the bike taking in the landscape and not have to worry about blowing by a turn as I was gawking at the stark surroundings. And, thats not to say, the road I'm on wont twist or turn and fork, it could, but you know just stay on that road and your good as the GPS counts down to the next turn.
But a route does require more pre-trip work.
So, how do you share a route between GPS's that have different map-set versions ? This is where the work comes in. Like I said earlier, you cant just grab someones track-line and run it. You need to create your route. So you email your friend your route made on a 2009 map-set in Map-source but he runs 2012. He opens it in Mapsource which it will do no problem. He then has to re-create the route, simply going over the top of your route, in essence, copying it. And there is more then 1 way to do this. But once he is done, you both have the same route built on your own map-set versions.
Way easier, and me and my friends now do this, if you all have Garmins Lifetime Updates. Then your all on the same, latest, maps and sharing a route is as simple as sharing a track.
Clear as mud I'm sure
All I can say is we have been on many off road, complicated routes over the years, and we never loose time going off course or lost anyone in the group.
Its a personal choice, both work, it just comes down to what you are doing ride wise, and willing to do and want from your GPS. What I typed is just an opinion. Not right or wrong.
But to answer the OP's question, YES, you can make a route with mixed, mapped and unmapped sections.