As the owner of a newly minted 3DS I was wondering whether it's worth picking up Pushmo and Crashmo or just sticking with the latest iteration. Is Crashmo just an evolution of Pushmo or are they different enough for it to be worth finishing both?

I haven't played Crashmo but as far as I know it has some pretty important differences from Pushmo.

You should start out with Pushmo. It's a great game and it packs a decent amount of content for $7.

I'd say get both. Despite being similar in a few respects, both are different enough when it comes to actual puzzles that you'd be just as well off with either.
 
In fact, I'd say support them both, so that intelligent systems decides to make these games a trilogy.