I'm not using very dark juice, but I'm getting about a month out of my heads. I vape 4-5ml a day. Is a new head best, sure, but I'm fine with the way they are now too. I clean them once a week. YMMV. As far as cartos go, I don't like them all that much. A bit better flavor, but I find the hits very dry on the mouth and throat. Not a dry hit mind you, they just dry me out. I do like them on a standard eGo for when I need a very pocketable device, or for on a lanyard when I'm kayak fishing.
I started on the Nova after 3 weeks, I didn't find it at all hard to use as a relative newbie (I tried carts 2yrs ago). Replacing a head is easy, rebuilding, not so much. Pbusadrdo's video on cleaning and rebuilding the Nova is a great intro video. Finally, at $2.95 a head every 3 weeks I think that's pretty cheap.
What do I recommend for the newbie, 2 Vivi Novas and 2 cartos. Gotta have options and backups for your backups right?