I say both. SU is better but you can still get some good stuff from squidoo in my opinion. I build one lense per niche and every time I submit an article to the directories I also post on on my squidoo lense
They are used for different purposes: Squidoo - build SE friendly links to your site and get long term targetted click-through traffic by creating a lens targeted at a keyword phrase that is not too competitive but has good traffic - Squidoo lenses generally rank very well in Yahoo, and okay in Google. Also earn directly from your lens through Squidoo revenue share and your own affiliate links. You can use Squidoo to drive traffic to pretty much any page on your site and is good to use as part of a larger link building campaign. Stumbleupon - Drive quick, low quality (untargetted) traffic to your site in relatively large volumes (5-20k over a few days). Most visitors don't stay long, click your links or subscribe etc. Stumbleupon will generally only send traffic to your most entertaining and interesting pages on your site (as users have to vote for it). Traffic usually comes in bursts and dies after a few days - occasionally reigniting for short periods. Sometimes it will send regular traffic to particularly interesting pages. Bottom line.... You should use both!