Wow. So far Xerneas and Yveltal seem pretty much equally cool to me (although Yveltal is pretty awesome, Xerneas seems cool in a more restrained, dignified way), so I'll have to wait for typings to decide. My guess from the colourings is that Yveltal is Fire/Dark (Red front and black maney thing on its back) and Xerneas is Water/Psychic (blue body and shiny rainbow antlers). It'd make sense as it would make sure that both are more or less evenly matched in terms of typing, and since they said the types are going to be more inventive this time around then it'd fit in with that as there aren't many 'mon with these type combinations at the minute.
"Brilliantly Bodacious Bejeweled Barnacled Buoyant Boisterous Belligerent Barbaric Beastly Booty Brigade" - Fubz