I had the F650, not the Dakar - the Dakar was better off road.
I'd take a DL 650.
It'll handle good dirt roads with less rider effort than the BMW's and it has reasonable wind protection. For the road sections it's way ahead.
The wind on the F used to beat me to death, not a problem on the DL. 300km's was a long day on the F, 1000km and I'm still going on the DL.
The positives for the BMW shouldn't come into play on that trip
- better ground clearance - you can take road bikes on that trip.
- 21" front wheel, also a non event unless you have large objects to get over.
"A little off road" pretty much is what the DL is good at, it's not a real dirt bike, but if it looks like a road you could get one of the pretend 4WD's down it'll handle it easilly.
That said, either will probably do the trip, so if you really want the BMW go for it. If you weren't doing it for the fun of the ride you'd just fly there right ?.