I have both gmail and yahoo, I rarely use either and really only maintain them for offnet testing. I have noticed my gmail gets less spam, and I am generally happier with its UI.
As I migrated to Gmail over the last year I have to admit one of the features I like most is how it groups email conversation threads together into a single line on my inbox.
Gmail all the way, yeah it sucks when it's down (as it was yesterday afternoon) but the once in a while inconvenience beats out of dealing with all the spam via POP or IMAP. I like that I turn off server side spam filtering on my server and just use Gmail.