having used’em both for doing comps of purely rendered sequences (no mix of filmed and rendered footage) i clearly prefer shake. i checked both of these packages without any prior compositing experience and with my usual habit of ignoring the manuals.
shake: it’s fast, very stable, has a slick, very organized and thus very easy to learn yet customizable interface and the scene-graph works just great. there are other strong points like script-ability, huge userbase and possibly a lot more but that’s what i noticed during my compositing-struggles.
combustion on the other hand was slow, a memory hog, really not very stable, had this (to me) strange and cumbersome to learn interface and a scene graph that wasn’t anywhere near shake’s.
but yeah, it’s cheap in comparison.

