If I had to pick between only the two people mentioned in the OP, I'd say Flea. Primus has never been listenable to me; at least RCHP had a few good records.
On one hand, I can see why he'd say that. After so many years, things are going to change. It's just, I'm a stickler for old lineups. Then again, "Epitaph" was awesome, I have the DVD, so maybe the Priest is going to stick around a while yet.
I've never been the biggest fan of King Diamond or Mercyful Fate. Lots of people like them, that's fine, but I did try several times to listen to them and I just never picked up the habit. It's funny, because I do listen to black metal, which adopted a lot of the stage style from artists like King Diamond. Nothing personal, just not my cup of tea.