NYCBuffalo.
My #1 rule is always buy Legacy however as Lionel has not produced a RS11 since 2018 if you can find one at a dealer I would buy it however if factory repair is important to you there is a cut off from warranty repairs at Lionel. I don’t know exactly what the cut off is but you can all Lionel to find out. There are forum members (Alex M. GRJ etc) who can fix any Legacy engine if needed.
I cannot give you an opinion on MTH either now of before their reorganization but a lot of people like them so if they are producing a RS11 in a road name you want I would say go for it. Mike Reagan (spelling ?) has done a great job with their parts and there are people who repair them. Please do not consider this a recommendation as I only buy Legacy engines.
JohnB