Everything on a bicycle works together so you really have to consider everything at once.
I'll muddy the waters by talking about the relationship of the saddle to the bottom bracket. That's the factor that I work on first. If you are delighted with the fit on the bike that you have now, drop a plumb bob from the nose of your saddle and see how far it is behind the bottom bracket. Try to make that match on your new bike when the seat height's right.
When that's done consider the reach and drop from your saddle to the handlebar. Do whatever you have to do to get the drop right and then just buy whatever length stem will make the reach right.
If you have the saddle pushed all the way back now I doubt you're going to like a bike that has a tt that's almost an inch shorter.
Last edited by Retro Grouch; 04-30-11 at 10:15 AM.