Many people ask this question with regards to what size tripod legs/base they intend to order, so I am going to give you my personal thoughts on Sitting vs Standing.
You always want to try and sit comfortably if you are going to be glassing for any length of time (15 minutes+). Your glassing will simply be more effective and consistent, when compared to glassing the same piece of country while standing. Sitting really allows your body and mind to relax, which will give you the ability to strategically pick apart a piece of terrain. While standing, you simply aren't as relaxed, therefore you glass much quicker and less strategic.
So what Tripod length legs should I get?
I would get a tripod that is long enough to stand with.
Why? You just said sitting was better...
The reality is, the lay of the land and the animals you are hunting should dictate your glassing position and strategy. I find myself standing 30-40% of the time, but it's not by choice. When I am Desert Bighorn Sheep hunting I am going to be glassing 99% of the time from my butt because it's so open with big elevation changes. While late season elk hunting I am going to be glassing 60% from my feet. Most of the time it's because the pinion junipers I hunt only allow small glassing windows, so I have to move around to pick apart the country.
Below is an example of a big bull I glassed up in a small window while standing. Elk are typically easier to see when compared to most ungulates, because of their size & color. With that said, most of the bigger mature bulls will stay in the thicker country which often only allows for glassing while standing.
I personally would recommend the Tall Tripod Legs/Base for almost everyone, because it's going to allow you to sit, but also stand and glass with out an extension (unless you are over 6' 2"). When I am hunting I only want to carry x1 tripod. I need it to allow me to do whatever it takes to find the animals I am looking for, no matter the terrain or species.