Great operas, written about 25 years apart and still demonstrated the musical characterisation and genius that was of his creativity, not seen again until perhaps Mozart, Wagner and Verdi.
Anyway, to answer your question, Rinaldo is easy: The Academy of Ancient Music/Hogwood. Title role by counter-tenor David Daniels. Great recording, universally recommended. (I have three versions of Rinaldo, and the other two all feature mezzo in the title role).
As for Ariodante, I'm afraid I can't think of any (as far as HIP is concerned, and I don't really care to listen to Handel on non-HIP). My version has Anne Sofie von Otter taking the title role (Les Musiciens du Louvre/Minkowski).
Historically though, it was a fact that when the leading castrato was not around for a performance, Handel was known to have substituted the castrato with a female mezzo/equivalent. Personally, I prefer the mezzo/female equivalent because it is a fuller voice and able to encompass the vocal range, which counter-tenors sometimes struggle with, especially at lower registers. The historic castrato and the mezzo/equivalent were never falsettists. So I'm going on purely musical reasons.
Good luck with Ariodante, keep us posted!