(opinion: i've spent a year writing Dropwizard based apps, but only two evenings writing Spark based apps).

I think that Spark comes at the cost of customisability - 2-way SSL is a chore, you have to parse your own message bodies, you don't get straightforward logging out of the box - there's a base set of features and functionality that I couldn't find in Spark.

The thing that impressed me about Dropwizard is that you can get in perhaps 50 lines of boilerplate a really solid core behind an application - with Spark you can do that in perhaps 20, but you are somewhat limited beyond that.