The diesel overall will have a lower TCO.
Marine diesel is usually 25-30% cheaper than marine gas, even in states where diesel is more expensive at your corner gas station (don't pay attention to gas station prices, marine fuel prices have a different tax structure and can be *very* different depending on the state). Even with the same miles/gallon rating, you'll spend a lot less to power a diesel boat.
Re: maintenance, as @BB marine said, the maintenance ends up being something of a wash. Also, some maintenance will require a haulout which adds expense (sometimes quite a bit, depending on yard availability where you are). Having maintained both diesels and outboards (as well as land-based diesel and gas engines), I think that for a DIYer, diesels are simpler overall.
I do think there is also something to be said for diesels generally being more reliable, breaking down less frequently, and being somewhat "easier" to fix when they do. There isn't much that can go wrong with a properly maintained marine diesel, and the vast, vast majority of the time it's a fuel problem. Gas engines, on the other hand, have a lot more that can go wrong even if properly maintained. Boaters (especially sailors) who put a high premium on reliability and longevity almost universally favor diesels. The service life of a 4-stroke outboard, even well maintained, isn't generally perceived as more than 5-10k hours (I say "perceived" because I've never seen data, only spoken to mechanics). But a there are diesel engines with 30k+ hours still running fine, and properly maintained 10k hours is often seen as the floor. This matches passenger cars, by the way: in good diesel sedans, the engine is usually the last thing to fail (I've worked on a diesel Mercedes that had 500k miles on the engine).
I would say on the plus with the gas boats, the storage is truly bonkers. Move the engine outside the cockpit, and all that room becomes wet storage. I'm a little envious of that.
