No scientific data here - but I rode them both in less than a year, and really can't tell the difference. Maybe someone else will actually have one of those cool statistic books that will give us the numbers!
I personally think the WDW Splash is a better attraction BUT the drop at Disneyland feels more thrilling, perhaps because of the inline seating VS sitting side by side at WDW.
I think the whole ride at Disneyland feels faster. Probably because the logs hold fewer people and are narrower, they must move along faster. I like the one at Disneyland better because our family usually gets our own log which is better for the picture and it doesn't smell like mold or whatever that awful smell is in the one in Disneyworld! The drops feel about the same to me. I also don't think you get nearly as wet on the Disneyland version.
I'd say the drops are about the same, but my vote goes to the WDW version which to me has a much better "show" - a coherent story line, slow enough that you can see it, and seemingly more show elements.