So, let me answer my own question :-)
Drove down to Eilat via Mitzpe Ramon at night, back via Dimona (Arava rd) during the day (morning).
In retrospect, the other way around is better.
The Mitzpe route has nice scenery, but a lot of sharp turns and and narrowish winding roads. The Arava road, on the other hand, is a ong stretch of fairly straight road with short sections of gently winding uphill and downhill stretches. It's thought to be more dangerous - as more truck drivers use it, and it's easier to fall asleep on a long, straight road.
I preferred the Arava road myself - easier driving.
Both routes took 5 1/2 hours each, with about an hour of stops included.
I eventually found some info on this, but in Hebrew - so thought I'd post here in English for future reference. If any....