University I rejected is asking which other one I chose over them; should I tell?
I can't think of any harm in telling them, and in the medium term, this isn't a secret, because it will certainly be clear by the fall where you've gone since your name will probably be put on the department website.  If you're uncomfortable, it's certainly fine to wait a bit until you've finalized things at the school you accepted; you don't even need to tell them you're doing that---just wait a bit before replying to the request.

It may help to know that it's standard for schools to want to know where students who reject them are going, so there's nothing unusual or suspicion raising about the question.  It's useful information because it helps them assess their program and admissions practices.