British Whose Line >>>>>>>>> American Whose Line.
Ehhhh... I'd say Drew Carey Whose Line > British Whose Line >>>> Aisha Tyler Whose Line. It's been a while, but I remember the British version being fairly inconsistent, and most of the best episodes featured Ryan Stiles and Colin Mochrie (and occasionally Wayne Brady). The best #4 cast member was Greg Proops, and Laura Hall >>>> Richard Vranch.
The BBC version had a much better talent pool to pick from, and utilized it well. The US version basically had a cast of 5 with two of Proops, Sherwood, and Brady teaming up with Mochrie and Styles. The BBC had Mike McShane and Josie Lawrence, who may well be the best there is at this, along with Mochrie, Proops, Styles, John Sessions, Stepehen Fry, and the great Tony Slattery, who was probably my favorite of the lot. While the US version got redundant, the BBC was constantly putting together different groups.
Also, I prefer Clive Anderson's subtle wit (turns out he was Footlights, which doesn't surprise me). As for the musicians, Vranch struck me as far more versatile. He probably played ten different instruments during the show's run.
Interestingly, I've got all howevermany seasons of the BBC on my travel laptop, because it's something you can always put on and enjoy. I watched six or so episodes on the flight home last month, and enjoyed it a great deal. When it really works it's fantastic. This one really had me cracking up on the flight.
https://www.youtube.com/watch?v=rDU05dUZNAM