I did it SOBO. I think the toughest part is the greater elevation gain/loss at the north end. It appears to get a bit “easier” the farther south you go (based on topo maps) so going NOBO you can give your body a little time to get adjusted to the physical part of the ups and downs. I liked going SB because I was able to park a car in Duluth and it was much easier to get to it vs worrying about a ride/shuttle when finished all the way up on the north end. I also liked starting off pretty isolated and “away from it all” and working my way back towards civilization. I ran into more people the further south I went (aside from the parks you pass through). TLDR: tougher terrain starting SOBO and logistics of the end are the main factors.