I think "for" is more natural. Possibly I'm influenced by a line from the well-known movie 'Wizard of Oz' (She had to learn it for herself) In my opinion, the "for" makes the discovery seem more deliberate. It's your actions that will make you find out.

They're both natural. The 2nd ("I thought it best to let you find out yourself") is slightly more casual with the dropped 'for'.