I own Maokai, and I'm a few days away from purchasing Trundle, so I'll be able to compare them soon.
But as of now, Maokai is a very strong jungle, very strong ganker, and very strong tank, with a huge aoe defensive cooldown for his team. His poke is strong, his initiate is great, and that knock up on a short cooldown is great for interupting Kats and Nunu ults. He's a little tough early on when learning him, but once you get the hang of him, he's amazing.