I think that the Japanese voice acting is just slightly better. To me, they sound like they put a bit more feeling to it.
For example, watch and listen to Vayne's attacks in the English version of FFXII. (Find a video on Youtube, play FFXII, etc.)
Now compare them with this:
YouTube - Final Fantasy XII Final Boss
Like I said, it sounds like the Japanese voice actors put more feeling to it. Especially when comparing the English and Japanese versions of Ascension, Gigaflare Sword, and Teraflare. I don't know if it's just me, but there are parts in those attacks of the English version where I can barely hear Vayne's voice actor at all.