Thirty-five hundred miles on a rear tire isn't too awfully bad. That's what I got on the stock Bontrager Race Lite Hardcase tires that came on my Portland. If you like the tire, buy another one.
Advice I heard very early in the game--and that I've practiced since--is to buy tires in sets of three. The rear always wears at roughly twice the rate of the front, so you'll already have a replacement. Plus, in case of serious damage, you already have a spare.
So buy the third tire of this set and save the decisions for another 3,500 miles.
Sounds like sage advice. Plus I only spring for one tire this time around. However, I never seem to be content with the status quo so I will be looking for that ultimate tire during the next 3500 miles.