There are some similarities, but frankly, more differences. crowdSPRING projects are, on average, much higher in value. There are typically many more $1000+ projects, for example, and often, $5000+ projects on crowdSPRING.

crowdSPRING offers graphic and web design projects (99designs does too). But crowdSPRING also offers industrial design (products and product packaging) and copywriting projects.

More info about differences here: http://bit.ly/whycS