I think when it comes to bang for buck the Surly make for a fairly easy choice.
The mounts look a little fussy but for shear "bolt on" choice the bikes are hard to beat.
IMHO they are a little more versatile than the Fargo.
Not better, just a little more versatile.
If your son likes his Troll as much as I'm enjoying its big brother (Ogre), I'm sure he'll be very happy.
The different width tires the Troll will accept is incredible.
Happy Spinning OP (and son).