I had an Aubergine 620 for several years; it is a fantastic bike and one of my favorite past-owned bikes. It's "limitation" to me was that it's a single, because of the to/from ride distance to the places I like to ride and lacking as even a weekend sport touring mount (doable, but tiring). I sold it and bought a 950SM in 2006. While there are some aspects of the Duke I sort of miss, the 950 is close enough to the single benefits with a lot of twin upside.
Pricing seem to be in the right ballpark-- 620 Duke pricing has been pretty constant for the past 10 years, and 950SM depreciation has generally been heavy.
the 950 has had at least 1 important tech bulletin that should be done- the starter limiter (aka the klacker), there are possibly others I've forgotten. There's an ownership thread in Road Warriors (where this thread should be anyway) at http://advrider.com/forums/showthread.php?t=153375
that has everything you'd want to know about the 950SM.