[*Varda*]

I don't think anyone can say Aragorn made the 'wrong' decision to go after Merry and Pippin. Although we know what happens next, so it's harder to imagine not knowing, Aragorn knew his destiny lay towards Minas Tirith not Mordor. His path went towards Gondor, not Mordor and he knew it. There were even prophecies about it, such as he had to walk the Paths of the Dead.

It could have been equally dangerous if Merry and Pippin had been taken to Saruman and everything had been found out about the Quest. And Aragorn probably felt they both had some part to play before the end.

And if Aragorn had gone with Frodo and Sam I'm sure the whole thing would have failed. I don't see how Sauron couldn't notice Isildur's heir walking into Mordor. And Minas Tirith would have been destroyed for sure.

But basically, even Gandalf didn't know what path they were going to take. All Aragorn could do was to make an educated guess.