Generally, cutting wood works best with straight two flute bits and plastic with single flute up-cut spiral bits. Down cut spiral bits can provide cleaner cuts in wood and may be worth the extra cost in some applications. Compression (up/down spirals) are only beneficial when the first pass is deep enough to get past the up spiral portion - not possible w/ the X-Carve.

Inventables bit articles:
Carving Bits 101 - Bit Basics
Carving Bits 201 - Feeds, Speeds, and V-Bits