I can't figure it out. I'm pretty sure a drider has a human torso and a spider lowerbody (ala 8 legs and spider abdomen), but I can't narrow down what constitutes a spider-morph.
Drider could be said it's a spider with his head replaced by a human top half (think in simpliest way said) so not have normal lower body half aside chitin on legs and abdomen sticking in place of tail.
Drider is a very specific creature, being a DnD monster popularized by centaur-fetishists and being hooked up with Drow. A spider-morph is anything with spider bits, even if they're bipedal.