YouTube is more commercial. YouTube is more for 'less serious' efforts.
Vimeo has a cleaner interface, better functionality (if you upload a file and want to replace it, you can do that without having to delete and all settings, view counts, etc. remain in place) and is the more serious of the two. Vimeo doesn't allow commercial postings and it doesn't allow using its servers as a hosting source for video you want to embed elsewhere (e.g., stock clips shown on your website).
Vimeo's got some significant names associated with it (David Lynch, Roman Copploa, Vince Laforet, Morgan Spurlock) but YouTube recently announced a project involving Ridley Scott. If that kind of thing matters.
Vimeo doesn't have a clip length limit but it does have a size limit. Max 2GB/clip. 5GB total upload limit/week with a Plus membership (~$60/year).
I've got stuff on both but prefer Vimeo, for whatever that's worth.