I've been to London and Paris:
London: Outside tourist areas (still downtown though) it's just if not more messy then Toronto, in terms of litter on the ground.
Paris: Is clean in most places! I'll give them that.
Both I find are rather dark and dreary, London has a severe lack of green space, and the acclaimed parks such as "Hyde park..." aren't much to write home about to be honest ... I'd take High park over that.
Paris is better but still suffers from the same problem. I'd take London over Paris any day though I find it has more life although the CBD and surroundings are disserted any time after 5pm in both.
Either way the both have great architecture, amazing density and a wonderful transit system (below ground at least...) I think that comes with being around for so long though ;-).