It's a personal preference thing...
I strongly prefer Southbound...
Since I start with a Cruisetour, doing the land portion first means I start with all the getting on and off trains and buses, checking into a different hotel every night, eating in restaurants...all the difficult stuff...First...
Then, after a week of tooling around Alaska...whn I am good and tired, I get to board the ship...unpack for basically the last time, start eating those magnificent meals...and really get pampered...and relax...
My flight THERE is the long one...but I can deal with it easily, knowing what I have to look forward to...
And then, at the end, when I am tired out and looking to get home, I get the short flight...