Espar, hands down. It will pay for itself in just one winter for most people, and in a matter of a few weeks if you're not insulated and are idling. Insulate your van.
The D2 will keep you warm and toasty in the most cavernous of Sprinters, BTW. RLENT has one, no bulkhead, stays warm, sleeps nekkid. (But, he's insulated. Insulate your van. sheesh.) The D2 will just have to work a little harder than the D4. The D4 puts out twice the BTU's for the same amount of amps and fuel, and puts out twice the airflow, so it'll keep you warm the same as the D2 but it'll do it while working less.
When you get a new vehicle, you can always take the Espar with you to the new one, assuming it uses the same fuel as the old one. They make both gas and diesel models.
I have the D4. Would never do another winter without one. Been there, done that, got the sweatshirt.
Slow and steady, even in expediting, wins the race - Aesop