Who imports the more accurate Southern PS4 - Lionel or MTH?

I have to answer both. Southern Shops made so many alterations to the 4-6-2s, in particular in the Eastern Lines Shops at Spencer and Pegram (Atlanta) that its easiest to avoid trying to sort out the many "hybrids". Over on the CNO&TP and AGS  the Master Mechanics made many modifications notably for the Rat Hole Division units. Toward the end of steam on the Southern all of the "Harrison" Crescent Ltd and Queen & Crescent Ps-4s were moved to Spencer---some even got "Spencerized". Of course the Ps-4 # 1380 "Tennessean" was streamlined at Spencer:  (1) it did not have an Elesco Feedwater Heater on top of the smoke box in the way of a shroud install and, (2) it was already at Spencer undergoing a major "shopping" when Washington HQ made the streamline decision.

Past series of 4 or 5 articles on the Southern Ps 4s in TIES Magazine is the best source.