Last year, I was doing a similar comparison. With kid #2 on the way, I knew that my trust '94 Ranger wasn't big enough and I didn't want the bulk of a full sized truck like the F150 SC. So, basically my short list came down the the Tacoma, Frontier, and Sport Trac. The big negatives that the Sport Trac had were that I knew a new model was coming out, its bed was by far the smallest, and it was the least capable off road of the 3, and its drivetrain warranty was the worst. I ended up going with the Frontier, IMHO its exactly what Ford should have done with the Ranger. I've had it about 14 months and 8k and my one glitch that I had was that the guage cluster had a loose wire that was causing the guages to "jittler" on start up (lose wire = bad connection = guague doing a reset). Otherwise its been running flawless.
Had the new SportTrac been out, it would have been a a tough call. Ford definately made a decent amount of improvements to it and its about time they upped their warranty too.