I switched to Christian as Chosokabe as soon as I had the whole island. European cannons and matchlocks are devastating and fun to use. Nanban trade ships rule the seas (cannons). You get an extra trade slot in your home province from the Nanban Quarter.

Cons are: no warrior monks, everyone but Christians hate you.

A note: don't switch when your lands are all Buddhist unless you can afford the garrisons to ride out the storm. It seemed like unrest gets steadily higher until it peaks out at 50/50 Christian/Buddhist, then starts going back down. I was unprepared for it and it was a little frantic trying to keep my provinces in line.