Considering #18 gave him a greater boost when she allowed him to gain his Perfect form than #17 did for his 2nd form, I doubt which one was absorbed would matter strength-wise. They were just pieces to the puzzle that were already designed into his structure and not additive power boosts like with Boo's absorption, so he'd be just as strong. The main change is that Kuririn wouldn't hesitate to use the remote, so #17's destroyed, Trunks or Vegeta kills Cell and everything becomes peaceful. #17 and #18 would be revived afterwards on Kuririn's request, though the bombs would need another year to be removed, so Kuririn and #18 likely don't become an item until a year later.
As for what it has for the rest of the plot, Goku and Vegeta would still be stronger than Dabura and without Other World training to open up a huge gap, Vegeta likely wouldn't choose to go Majin. Boo's averted and Super goes near enough the same as it currently is beyond minor changes like fusion only becoming a real option during the Future Trunks Arc. Assuming Vegeta still chose to go Majin though, Goku wouldn't have SS3 or Fusion, so everyone is killed, Ultimate Gohan later arrives and kills Fat Boo and revives everyone with the Dragon Balls, so beyond Boo/Oob's absence, not much changes.