Two stroke motors aren't nearly as fuel efficient as four strokes, although the newer one's are coming close. Why do you think the companies are spending so much on developing four stroke outboards (other than the fact the two strokes will be outlawed soon)? I had a Formula with twin 350's that didn't burn much more than my 18 foot fishboat with a single 175, not to mention you have to buy oil for the two stroke.
In my opinion, outboards are much more reliable than I/O's. I had my last one for thirteen years and only replaced a trim pump. It still ran perfectly when I repowered. If you happen to puke one, they are much easier to swap out.