To the OP, how tall is your son?
My son will turn 9 on January, and at about 55 inches tall he's right in the height range between 24" and 26" wheels. He has a 24" bike with 13" frame, and just bought an old 26" bike with 12" frame to covert to SS MTB for him (I ride SS a decent bit, and he wants to try it).
At 74 pounds, most kids forks probably won't compress too much for him, maybe with a big root or rock, but on "mellow" trails you mention it probably won't do much except weigh him down (most kids forks are very heavy). So, for your stated purpose I'd probably pick the Trek of the 2 you mentioned.
If you were tackling even mildly challenging singletrack MTB trails, I'd say err on the side of 24" wheels. However, if you are sticking to pavement and really mellow trails (rail trails, crushed gravel, etc.) it could be o.k. to move him to a small 26" depending on his height.