I actually had the same question a while back and asked Art Feierman from projectorreview.com. His response was:
The Marantz[v12s4] definitely has better build quality, and is probably a bit brighter in best mode, but the Marantz did not have exceptional black levels, at least not by today's standards. Of course it is lower resolution than the 1080p projectors. Personally, I'd definitely pick a new Panasonic - or maybe the forthcoming Epson 1080UB, over a refurb Marantz. Remember, the 12S4 was one of the very first Darkchip 3 projectors. I'd also expect the Optoma HD81 (which is more money) to also be a competitor. Lastly BenQ's W10000, someone emailed me, is now available under $3K, or at least I seem to remember someone telling me that - unverified.
Also worth considering, in terms of the Marantz - would it come with a new lamp, or are you going to find out, perhaps that the lamp has half its hours gone (some company's will set standards for refurbs, of no more than 500 hours on a lamp, others don't pay attention. I certainly don't remember, but I'd just about lay down a bet, that while the Panasonic's replacement lamp, like most "mass" products is around $400 retail. I suspect that Marantz will get $500 - $700.
Anyway, that's my two cents. Happy Holidays! -art