They are both ways to save and record wonderful bits of information you experience as you cruise the web and they both have browser-side gizmos for users.
The interfaces are radically different, though I suspect youll get more link and SEM value from Deli.cio.us in the short term.
Heres StumbleUpon vs. MySpace:
Both have profile pages that show personal interests and photos of users
StumbleUpon is built around an information and entertainment discovery recommendation experience where profiles lead back to discovery, while MySpace is built around the profile viewing and community communication experience.
It would be wicked if SU and MySpace could converge. Innit.