Actually, what I'd suggest as a first purchase, to get used to Polaroids and timing the development and the whole pulling and peeling operation, is Type 667. It's the least expensive (I've seen it online for $18 for a twin-pack, 20 exposures total, and free shipping for orders over $50), and the fastest (ISO 3000), though it's B&W and not subject to most of the manipulations and special processing that can be applied to other emulsions like 669. Type 665 is one of the most expensive; it produces a reusable negative, but (unless rumors are accurate in indicating a recent change) won't give a good print if you expose enough to make a good negative, and clearing the negative after processing is something of a pain to do in the field (plastic zipper bags full of 20% sulfite solution are all sorts of annoying to handle, carry, etc.).
For color, 669 is best for manipulation (emulsion lifts, image transfers, etc.), but 690 gives better color (to my eye) for conventional prints.