I would have to say the 880, 3120 or any other 120+cc saw. I think that as emissions requirements get tougher and engine improvments from new designs get more power from a lighter smaller power head we'll see a 100-110cc range replacing the 120+cc saws. Just my guess but I think there may be a market out there for a saw with 880 power in a slightly enlarged 660 package. At that point the big saws of today may be like the 090s and 125 macs, a nich market of collectors looking for a good saw and willing to pay to own one. That said I doubt the return would be any better that a savings account would pay over the same lenght of time.
In my case I bought the 880 to use, granted I take good care of it and barring some unforseen accident, or the goverment regulating good fuel (and thats where they'll try to get us) I dont forsee loosing any money on it if I do decide to sell it in the distant future. I couldnt spend 1600-1800 on a saw and not fire it up