Can't help but find it most amusing that the so-called self-appointed experts here who have come out of the woodwork claiming the MTH model is superior have neglected to mention one minor point. Namely, the MTH model was last catalog'd in 2007 and is very likely now only available on the secondary market, whereas the Lionel Legacy versions (3 models, 2 road-numbers) are reasonably available NIB, especially UP #9000 in the classic, old livery paint scheme freshly coming off a second production run earlier this year (with many of the major dealers showing stock right now).
I tend to purchase whichever brand catches my fancy, so I don't really have an axe to grind here. But suffice it to say, MTH was absent from the U.S. steam market for almost two complete years. And during that time, quite a handful of Lionel Legacy steamers made their way into my roster, and it's nice to see Lionel's steamers have greatly improved their smoke performance over what it was about 5+ years ago when I would have easily given kudos to MTH in that department.
All this nit-picking about 1920's vs 1940's vs 1950's era modeling may come into play for some folks... or it may not. Whatever floats ones boat is a personal thing and very subjective. All of that notwithstanding however, unless MTH decides to re-issue a 4-12-2 in the near future (and anyone's guess there is as good as the next person's), the availability of an MTH 4-12-2 is on a whole different playing field than that of Lionel's -- at least for the current moment.
As for how well steamers like the 4-12-2 track... the long wheelbase will definitely look odd on sharp curves, much like you'd see when folks try to run 21" passenger cars on O-54 track. Whether we like it or not, this IS O-Gauge/Scale, and some products are simply meant to run (and look) best on O-108 or greater curves.
Just sayin'...
David