Yeah, Mahinmi definitely has a higher ceiling than Splitter. The nice thing about Splitter is he was a plug-n-play option. In fact, Splitter is probably more ready for the NBA than Oberto was when Oberto first came over. However, like Oberto, Splitter doesn't have huge potential. Splitter basically could top out as an average to a slightly above average center.
Mahinmi, on the other hand, is much more of a wild card. He's not ready right now but I think the Spurs have to force him to be ready. As Duncan ages, there will come a time where he can't be counted on to clean up every mistake as the other bigman on the court for the Spurs watches. That's where Mahinmi's athleticism could really come into play.
Mahinmi could either bust out of the league and be back in Europe in two years or he could be the missing piece in the middle that has been empty since Robinson retired. I think it's 50/50 either way right now.
Summer league will be an important time. Pop believing in him early on and giving him playing time will also be vital. I don't care if he sucks for the first three months next year, he has to get his 15-20 minutes per game. At worst, the Spurs need to figure out ASAP what they have. Burying him at the end of the bench for another year would be a mistake, IMO. Splitter would have been nice because he would have been able to come in and start and give Mahinmi time to grow. It looks like the Spurs don't have that luxury so it'll be time to let Mahnimi sink or swim to begin next season.