I've tried both Pai and Khao San Road numerous times, and while I want to give Pai 4 stars I've just always liked my experiences at Khao San Road. Here's a breakdown of why:
Food: Hands down, the pad thai at Khao San Road is more flavorful and consistently yummy. The one at Pai was a little more mushy (overcooked noodles?), and last time my friend received a clump of noodles all stuck together as if it had been sitting out for way too long. Totally not impressed by the noodle dish that's sold for $17. Moving on from that - the green curry at Pai will always win in my heart. It tastes authentic and is served in a coconut shell - ask for spicy if you're into that because the baseline for this dish is very mild/medium! I've heard that the Khao soi is also really good at Pai, but I've only had it at Khao San Road so can't comment.
Drinks: Pai has a more extensive alcoholic drinks menu, and during the Raptors post-season playoffs games they even had Raptors themed cocktails. Not sure if that's still a thing now that we lost again. HOWEVER, the Thai iced tea at Khao San Road is much yummier and creamier. The one at Pai is filled with ice to the point where it feels like you're drinking a slushie. Yuck.
Service: ok, this is arguably the most important part of dining out (besides the food). My experiences at Pai have always varied... from good to sub-par. Last time it took 45 minutes for my food to arrive on a Thursday, and all the restaurant did to compensate was give us ONE dessert to share amongst FOUR adults. Why even bother to attempt to act like you're compensating at that point?
Pai, there was a time when I really loved you but now Khao San Road just really has my heart. The only reason I'd come back to you is for your green curry!