I don't think it matters which one you use, at least as far as the synthetic oil is concerned.
I don't remember which silicone I used on my differential last, but it was probably whatever was in my toolbox. I have had a leak or two with petroleum oil, but none with Amsoil. I don't attribute this to the oil. I think I have just finally learned how to do the job correctly.
One important point about silicone RTV: IT GOES BAD. (No, I don't mean that it starts to do drugs and steal things. ) Old silicone RTV will not set up. I found this out the hard way, with a bathroom job. I'm glad it wasn't something on my beloved trucks. In my case, GE replaced the sealant with some fresh tubes and paid me for my time to re-do the job. This was after I chewed the guy out about not putting expiration dates on the tubes.
The rule: If you don't know for sure that it is fresh, buy new silicone RTV for any critical jobs.
Good luck.
Loren