I have both the MTH & USA Big Boy...Both great locomotives..
I also run both 1/29 & 1/32 rolling stock but I won't put a 1/29 boxcar behind a 1/32 locomotive without having some buffer cars in between say like 20 MDC hoppers. Some cars are kind of in between the two scales and will actually work very well behind either Big Boy. On the same note I won't put a 1/32 boxcar behind a 1/29 locomotive.
I actually run my MTH Big Boy more than the USA...the MTH version is a heck of a lot easier to handle!
I also run both the MTH Hudson & the USA Hudson though I have to admit that the USA version gets much more run time than the MTH esp. since I've DCS'd it...
What's kind of neat about the new MTH Triplex is that it's actually a little taller & wider than the MTH Big Boy.
The only bad?? thing about buying a new MTH Big Boy is where you going to find one?? All the dealers I deal with have been out of stock for over a year & MTH is out of stock. Rumor has it that MTH is going to do another production run..maybe. The new ones that I've seen on Ebay are bringing more $$$ than what they sold for.