Sure, it's situational, but most folks aren't using a CUT on their lawn, so that probably narrows the discussion down to SCUTs...at least when we start wondering if lighter might be better.
Well, I never said diesels would't cost more, I just said the gap would narrow. Generally speaking, nobody makes a gasoline engine, with the most modern features that are required to really get good efficiency like the folks in this thread are talking about, in a package truly equivalent to the diesel engines we're using as a benchmark. If you look at the engines used in generators, or inboard boat engines, or medium duty trucks and compare gas to diesel, it's pretty much a given that the diesels are intended to last longer, and survive more difficult duty cycles. In other words, if you buy the version with the gasoline engine, you know it's not going to last as long as the diesel, and that's why it costs less. If someone wants a 40' sport fishing boat they can get it with gasoline engines, or diesel engines, but nobody expects the gasoline engines to last anywhere near as long, or be anywhere near as fuel efficient.
I would leave out references to aircraft piston engines....they're a completely different animal. They're designed to be as light as possible, and require extensive maintenance on a regular basis....and they still fail with regularity.
As has been said many times in this thread....comparing modern diesel tractor engines to pretty much any known gasoline engine is going to be an apples-to-oranges comparison. If someone decided to make an engine that could be expected to perform reasonably as well as it's diesel counterpart, and have a similar service life, the cost difference between the two wouldn't be nearly as big as we see in things like commercial light trucks, or boats, or anything else.
I see a lot of mom and pop landscape people running gasser ZTRs, but all the bigger places seem to be running diesels around here...SCAG, Farris, Hustler, Kubota and Deere.
I'm not sure what kind of finish mowing you've been doing, but I ran a 90" finish mower behind 40hp PTO and it was like it wasn't even there. Running my 87" flail mower behind 45hp PTO is far, far more heavily loading the machine, and even a 60" medium/heavy duty rotary mower put more of a strain on either machine than the 90" finish mower. I'd expect any reasonably sized tiller, snow thrower or even post-hole digger would put more of a strain on the machine. A couple of 12-24" blades going through grass shouldn't cause nearly the resistance that heavy brush, dirt, or snow would.