"Which sounds better, Merlin or Griffon?"
Merlin.
The Griffon has a kind of lumpy roughness which would be very attractive in a V8; but doesn't compare to the ripping-calico sound of a well tuned Merlin on full song.
And......
I hate to admit it; but the Packard Merlins sound a bit sweeter than the RR ones (judging by tractor pulls, the only place I've heard both at the same time).
How ever the absolute bestest, sexiest sound of 'em all is the Sabre, leaky clunk box that it was.