Well, mainly I was interested in how they compare in price.
But beyond that, sometimes MTH and Lionel will issue the same engine with small (or not-so-small, depneding on your point of view) differences in detail and structural elements. Like the GE Evolution ES44AC. The first picture is the Lionel model, the 2nd MTH. (Or at least that's how they were labelled on eBay.) The horn in the 2nd photo looks a bit larger. There are what appear to be grab irons running up the leaf in the 2nd one. You can see a bunch of other small things...variations in size and placement of screens, stuff like that.
- Mike