Electronic (net-available) version of either - I have both bookmarked, as well as some others that I use depending on mood, word, context, and initial success in finding. I admit I go to Dorland's first on most occasions.
I'm with Trops; Dorland's for the first look, online for all dictionary searches. (BTW, there are full, unexpurgated versions of Stedman available for free online. They're on physician-access-only sites, but I found it easy enough to get a client to set me up with a log-in. Ironic, though, isn't it--the guys making the megabucks can get the full digital unabridged Stedman's for free, but the peons making the pennies are expected to pay through the nose for it.)