Model Conrail in 1993 and have a fair number of NS run-through locomotives (plus CSX, etc)
or
Model NS today and have a fair number of CR patch jobs...
Take into account conspicuity striping, availability of particular models or likely future availability. Also rolling stock. 1993 means goodbye Athearn Ethanol tanks, Kato Maxi-IV's, etc... It means hello more colorful shortline boxcars though.
I am getting second thoughts about going whole hog over NS, given that Atlas will certainly have the whole C40-8* family out there relatively soon, which I've been begging for the past 15 or so years.
/ljj