Here's the thing. Youkai Watch is very, very entrenched in particular Japanese cultural elements. So it will be strong so long as the youth of that country (cause Youkai Watch is pretty directly targeted at them) remains interested in those cultural elements. But that does tie YW's fate to the popularity of the things it has tied itself to. If interest fades, a key aspect of the interest could also fade. Pokémon, on the other hand, while still having cultural ties, has its own internal "culture" or mythos so to speak that can sort of stand on its own. This gives Pokémon a sort of independence that YW can't have, which is why Pokémon will almost certainly outpace YW consistently outside Japan. There are a lot of variables outside of this one to consider. However, I think the safe bet for the long run is Pokémon.