Prose With a Dash of Poetry; Conversational
Middleton does some prose here, but he's not turning in his poetic license. Many plays of the time period are in a poetic pattern called blank verse. Middleton doesn't so dutifully follow the rules of blank verse, though, so it's probably reasonable to call this play prose. That said, Middleton certainly drops into poetic rhythms and even rhymes from time to time (like at 1.1.81-82, where he rhymes "displaced" and "disgraced"). Way to play both ends against the middle, stylistically speaking, Middleton.
We should also point out that this play is written like a, er, play, and as such, it's filled with conversation. This isn't a piece that brims with descriptive monologues, but instead prominently features back-and-forth between characters. After all, you can't plot to kill someone without really discussing it with your sidekick.