Dan Sung has written a very detailed comparison of the five main cloud storage competitors. And I agree with this fragment of Dan’s conclusions:
(…) The most powerful approach of all, though, is probably to use these things in combination. Use the free 25GB from SkyDrive for your photos, Google for your docs and mail (or perhaps more Windows Live) and Amazon for videos seeing as there’s no file size limit. Dropbox is the one to use for convenience. It’s the best syncing tool of the lot and given that storage on the service isn’t cheap, use it as just that – a tool.
Possibly the best advice of all is to use them all. Use them all up until the point where you have to pay for them. That way you’ll max out your storage and minimise your cost but perhaps use each service for just one type of media. That way you might actually be able to keep track of it all.