Any less than 2 R9 280x or 7970 I would say counts as small for the purposes of the 2 p2pools - so below around 700KH/s you'll be better off on the new p2pool2, and certainly above 1MH/s I expect you'd be fine on the bigger original one. Also, if you're at the lower end of the hashrates applicable to the older/bigger p2pool, you'll see gains in share-rate as some of the small miners move over to the new one and the share difficulty drops.