FLV is a container, DivX is a codec.

FLV usually contains AAC or MP3 format audio & H.264 or some proprietary video codec.
DivX is usually used with MP3 audio or AC3 and put in an AVI container but can also be used with H.264 afaik.

So actually both are as good but FLV is probably more widely supported if you're streaming it (since you'd likely use a flash player).