Both are good services, but I really like the integration provided by Ubuntu One. For example, when I write an e-mail in Thunderbird, when I go to attach a large file, Thunderbird will offer to upload the attachment to Ubuntu One, share it and e-mail the link instead. Stuff like that really appreals to me.
The only complaint I've had with One is getting it to run on other operating systems (Windows and PC-BSD). It can be made to work, but I've found it isn't as smooth as getting it running on Ubuntu.