I ran both during the days of 3.x and out of the two, myself and my players preferred Scarred Lands.
I ran two campaigns with Wilderlands and no one was really impressed by the setting. I remember one player saying it was Greyhawk lite, meaning that it felt extremely generic and without any real setting hooks other than what I as the DM put into it. Another player felt that it was too much like 1950's and 60's fantasy. A little weird and not all that grounded. I see what they mean. After two campaigns we never went back to it.
Scarred Lands on the other hand I was a fan of from day 1. It was different from the usual D&D campaign setting and but no much that it didn't feel like D&D. In the early days I got a D&D meets Clash of the Titans vibe, and I would try and include that in what I wrote. I ran several campaigns on Scarn and they were popular. I lost interest after a few years when they started to take the tone away from the D&D/Clash o the Titans vibe in their books.
Of the two, I think Scarred Lands is better written and fits better with the D&D dynamic. It has life where I feel Wilderlands falls a little flat.
