Landscape all the way.
Hell, I'll even go a step further... Why the heck does anyone prefer portrait for printing out, other than the fact that that's what you're used to?
I recently printed out the Last Breaths of Ashenport adventure from Dungeon, which of course is in landscape format, and I found that the landscape nature worked really, really well at the table. I don't know how your tables are laid out, but at mine I've got ample horizontal space and limited vertical space. Landscape fits a lot better for me.
So basically, what it comes down to is this:
Portrait is best when you have ample vertical space and limited horizontal space. For example... uhh... Never. For example, you could fit more rows on your bookshelf and hold more books if they were all printed in landscape.
Landscape is best when you have ample horizontal space and limited vertical space, such as on computer monitors and most real-world situations I can imagine.
So why the hell is anything done in portrait mode again?