I have had both and OB is the way for me!
I/O: Pro Good on gas, easy to work on, cheaper to repower
Con still need marina for out drive service, always needs something as IMO a car motor was never ment to be in the water, nickle and dime you to death
OB Pro, very dependable, need little maintance, can be great on gas, easy repowering
Con, more to replace and when needed repairs can get costly if you need a marina.
Thats my opinion of coarse