I watched the MTH product video, albeit in low res, but to my eye the level of detail is similar. Both MTH and Lionel have appear to have cast-in piping on the boiler...
A low-res video tells you nothing about fine detail. The MTH engine does have separately applied piping; according to owners who have posted in this thread, much of the Lionel piping is cast-in. Some MTH detail such as the sand pipes is very close to the boiler and may appear to be cast-in on a video, but it's actually separate. I own the MTH version and used to have the 3rd Rail, and I can say for certain that both these models have a lot of finely crafted, separately applied detail. The 3rd Rail has a bit more and finer detail, but either of them has much more detail than the Lionel.
The MTH and Lionel whistles sound different, I have no clue as to which is the "right" whistle.
As several people have posted on this thread, the correct whistle for the 4-12-2 is the second MTH version, which did not come with the engine. You have to download it from the MTH website. MTH recorded this sound file in response to protests from knowledgeable customers that the original whistle, while it was a UP whistle, was not correct for that locomotive. I have the correct version on my engine.
To quote the immortal Robert A. Heinlein:
"What are the facts? Again and again and again – What are the facts? Shun wishful thinking, ignore divine revelation, forget about what “the stars foretell,” avoid opinion, care not what the neighbors think, never mind the unguessable “verdict of history” – what are the facts, and to how many decimal places? You pilot always into an unknown future; facts are your single clue. Get the facts!"
If you prefer the Lionel unit because it has Legacy and whistle steam, that's your preference. It's your money, and you set your own priorities. Others prefer more detail, a better whistle, and DCS, and others would rather accept some operational problems in exchange for the ultimate in detail. I don't see any "self-anointed experts" here telling people what to buy, just Forum members sharing their knowledge and experience, which is what this forum is about.