Welcome to 1789...

Personally, I don't think Saint-Just ever was Robespierre's disciple, in a meaningful sense. They were friends and would have continued so: the most obvious marker of this being Saint-Just's willingness to die with his friend(s) during Thermidor. I don't disagree that history might have been different had Saint-Just lived, but I don't think either of them would have ended up leading France, strictly speaking. Assuming that by leader you mean either some sort of strong president or a dictator, I know for a fact that neither of them--and almost no one of their generation--supported the former as an idea compatible with their principles and ideals, and as for the latter, it was just as manifestly against Robespierre's principles, if not quite as certainly against Saint-Just's. However, I think, assuming that Saint-Just was willing to entertain the idea of being dictator (which is, given the historical record, a pretty big assumption), he would have done it on 9 Thermidor in any case; he could never have attacked the Convention first, and he was too principled and proud to achieve it through the sort of back-room deals that Bonaparte would later employ.