While I don't think all of the fighters Ronda have fought are terrible there's a big gap in the level of competition regardless. GSP was fighting some of the best fighters in the world and utterly dominating them up until the end of his run (and he still won those fights mostly, had Hendricks up in the last fight but it was close and he got the nod). Ronda's opponents don't have the same reputation as guys like Shields, Fitch, Diaz, and Condit did at the time GSP fought them.
Ronda finishes more impressively and has a better highlight reel, so putting competition aside you could certainly give her the nod, but I have to include competition here when the gap is so large so GSP gets the nod for me.