Window trim is not usually subjected to much abuse unless you live hard in your house or you have pets that may claw at it when peering out. If you want a durable trim to withstand hard living, choose oak.

For both woods, a pre stain conditioner should be used to prevent uneven appearance. Two to three coats of polyurethane in satin will give it great protection.

Michael

Fir trim is indeed harder than most pines, but it's still a softwood. For interior trim it is normally more costly. As brushworks stated, interior trim doesn't get much abuse, so hardness/density isn't normally an issue. However, to me this would be the most important issue: aesthetics. Fir trim looks different than pine. The grain of fir usually has lots of heavy lines in it. Some people would love the look and others would dislike it. Looks great in a log cabin or in something rustic. You may want to compare the look of pine vs. fir trim to whatever the doors and windows are made from and choose a complementary wood species for the trim.