There are as many answers as there are records and Northern fans - I know old Wheelers that think the move away from RnB around 67 was a betrayal and that Motown is merely pop music made by black artists for white people. Conversely the boss (who started in the last days of the Wheel) is a total Motown obsessive and rates it more highly than a lot of out and out classic Northern Detroit indie stuff and hates a lot of the R'n'B I have that his older friends like.
I don't think there IS a line because where Northern-ish Motown becomes 'Northern' the edges are just too blurred - partly because I don't think you can define 'Northern' any more definitely than 'music played at venues that call themselves northern soul clubs' ...and once you get into 70s and 80s stuff (Motown or whatever else) well it gets REALLY messed up - I've seen people very recently STILL damning the Mecca's move into playing 70s and disco as completely wrong more than 40 years ago on here only a couple of weeks ago. The trouble is that about 90% of Motown interest globally is in the popiest chart hits by Diana Ross, Marvin, the Four Tops and Michael Jackson with zero interest in the supreme work of the also ran artists we revere - and that the most truly expert Motown fans are those with a love of soul largely rooted in a Northern Soul youth - even if they have moved away from it as their core interest.
I think nowadays most of us are old enough, mature enough and know enough to recognise that there is A LOT of soulful stuff that is great that isn't strictly northern (RnB, soulful house, disco, jazz funk, mid tempo, deep etc etc) - and a lot of currently/recently popular 'northern' stuff that wouldn't have been recognised as such 40 years ago.
Dx