Usually use the same model on the front and rear. But if I only have one left of a particular tire, I might mix them. On one bike right now I have a Hutchinson Globetrotter in back and Schwalbe XR on front, both 37mm wide. Both of those tires are out of production and they are the last of those two models that I own.
I typically run about 65 to 75 percent as much pressure in the front as the rear, thus my front tire with less pressure likely has better grip, even when the same tire model is used front and rear.
I typically want my best tire on the front, so my most worn tire is usually in back. Some people rotate the tires hoping that they both wear out simultaneously but I would rather have my best tire up front.
In the photo, this was my rear tire near the end of my last bike tour. I will probably use it again on my next tour on that bike, but use it on the rear where lack of tread is not much of a safety issue, my front tire looks much better than that one. This is the old Marathon Extreme, 57mm wide.
