Re: Sub for live sound
Go with the speaker. You'll have much more control over the tone and volume, and you may end up using it for other things down the road (vocal monitor, etc.). For a small gig, you'll get plenty of thump from a 15" woofer (whether or not the box it's in says "sub" or not).
EDIT: If space is not a concern, go with the Yamaha, because it's a 15" speaker (and not a 12").