+ has more "features"
- is more compatible (something like 86% vs 92%)
in either case, if it's a recent drive, it will read either format. unless you're planning on burning games for your ps2 or xbox, either format is fine (game consoles tend to like -). also be aware of the prices. last time i checked, - media could be had for about half the cost of + media.
personally, i'd just get a dual format dual layer (search newegg). i know there's a DL going for about $80 nowadays