The Traveler folds down smaller, the Mountaineer gives you more options for weight capacity and height.
For me, critically, the Traveler is small enough to fit in a standard international carry-on (20" nominally, but the inside is usually more like 18" long). The GT1542T and a Markins Q3T ballhead should be enough for a D800 with a 70-200 - you might get some wobble if you start using it at weird angles and such.
Just note that the GT1542T is very short (45" w/o center column), and you don't want to use the center column if you can avoid it. I'm tall, but I'm fine with stooping down to the deal with the height for the occasional picture (I rarely use the tripod). If I was using it all the time, though, I'd definitely want something taller - one good option is the Feisol CT-3441T.