Just an FYI - I would go with the self standing post only because your thread seems to imply your wall is allready set up ("leaning the 2x6 against a wall") - unless you meant the house wall.....which just means a lot of back and forth walking in my opinion..(far side being 24+ feet to the 2x6, 24+ feet back to wall to rest it again - those numbers add up too quickly)
Do NOT set your base rail and wall up before the ground is levelled, you'll just make it three times as hard on yourself.

Another reason I'd use a plated or tripod bottom is that the 2x6 can easily sink 1/2 inch into loose soil in a particular spot but right next to it may be dead on level where the tripod or plate method somewhat averages over a larger area eliminating the measuring in the aforesaid soft spot which could be a mis-reading.

about a year ago I wrote up an extensive thread on an AG pool install, where I mentioned that I used 2-3" compacted Screenings (crusher run) where my wall sits - this stuff is ideal for getting a nice perfectly level wall base and beats setup on dirt any day - of course, you have to go deeper at first, then bring the base level back up for your wall to sit on - if going that route, make sure you calculate that into your equation.
It does make levelling easier since excavation is done to a rough level and brought back to level using screenings which are WAY easier to work with.