Google's WMT is the best. It tells you what your back links are for Google. Yahoo will tell you what all your junk links are but they will not be counted in Google.
Don't put it like that. Both are state of the art in their own respective ways. If you wanna check what Yahoo has gathered, then go for it. If you wanna see what Google has gathered, go for that. Yes in Competitor sense you can easily check linkbacks for your competitors. No matter what, you still gather data from both of them. Thank you
GWMT is pretty slow to update though. Your rankings will change as an effect of new links it's found, but it will take some time before Google actually displays them in GWMT
For your own website google services is the best. for spying on other sites yahoo is better. for spying on other sites totally you should use them both!