I find it difficult to say actually (although that might be because of a speech issue I'll be having until my teeth are back in the right place) but because I can't say it I don't really like it. I can say Isaiah and I love Isaiah.
I've seen Isiah used and pronounced the same as Isaiah, which seems wrong to me. I love the proper prn. for Isiah, but I'd be afraid it would be confused with Isaiah (which I like as well).
I prefer Isaiah. I don't care for the pronounciation of Isiah, and I would be concerned about misspellings. Nothing wrong with it, though, just personal preference.