I think it might be minimally more economical to do it now, but 10 years down the road is a long time to sit with it. What if you change your mind about what you want up there? What if you need/decide to sell your house which would be essentially only partly finished?

Our plan: we'd figured out the basic layout of the upstairs. (You'll probably need some bearing walls in the interior, no? So figure out where they will be and what the room layout will be.) it'll certainly be easier to have a complete plan of the upstairs before starting, so you'll know where the stair will come down, and they builders will frame in the opening for the stairwell into the floor, etc. Personally I think I'd have the stairs done now and just have a closed door at the top... Our plan was:
-Have the builder do the exterior framing, roofing, window installation, exterior siding, interior stud wall framing.
-Since we DIY plumbing & electrical, we'd have carefully planned the rough-ins for those to correspond with the downstairs kitchen electrical and plumbing, and run plumbing stub-ins up through the floor and electrical up through the floor (that way the downstairs can be finished and you won't have to open up walls later to run stuff unpstairs!)
-We planned to leave the interior walls bare- just studs- but to insulate the exterior walls and seal them with vapor barrier. We felt that at that point it would be OK to leave the top floor unheated -or very slightly heated- until we finished it.

So, we'd have a finished downstairs, and a finished exterior. The stairway to the second floor would be sealed/closed with a door. Upstairs, the floor would be plywood subfloor, the interior walls just studs with plumbing and electrical rough-ins coming up through the sole plates but not going anywhere, and the exterior walls insulated and vapor-barriered.

I'm not sure I'd want to leave it like that for 10 years, though! What if circumstances changed and you had to sell the house or something? You'd have an empty shell.

And wi-sailorgirl's point about paying taxes is a good one. In my town that add'l living space would add a hefty chunk to the annual taxes :(

I guess an in-between solution might be to do the careful plans for the second floor now, and run the plumbing/electrical in the downstairs walls, and make sure your architect/builder knows that you intend to "pop the top" at a later date so that construction details can be planned to facilitate that later.