I'm in the same situation re: buying a second hand GPS.
It all depends how on old the Nav.II is? They come with a 2 year warranty, unlike the 1 year warranty on the 2610. I'm buying a used Nav.II privately, just over one year old. The remainder of the warranty is transferable as long as the seller can provide you with the original proof of purchase (dated of course).
I prefer the Nav.II because of the extra console with the 4 buttons, easier to use than the touch screen of the 2610. The Nav.II retains the touch screen though.