Here's a great piece of advice: learn how to tie a taut-line hitch:
Once you get the hang of it, you'll be able to tie off guy lines in 15 seconds. At that point, you'll never have to depend on any contraption or be constrained by their limitations.
Let's assume you learn how to tie a taut-line, and are therefore free to pick any line you want. Here's another great piece of advice: get two sizes.
First & foremost, get yourself some paracord 550. This stuff is universal, and you should be carrying 10' or so regardless as part of your emergency kit. (As the number indicates, it's rated up to 550 lbs, which is sufficient to either hoist someone out of a jam, or tie down a splint.)
As an added bonus, you can use the 550 for primary stays to either one or two trekking poles, branches, etc.
As yet another added bonus, since the 550 is woven with multiple internal threads, you can unravel those & use them as thread & needle (which should also be in your kit) to perform field repairs as necessary. (Including stitching either yourself or someone else.)
Second, get some 2mm (typically para 200) for all your other tie down requirements (A-frame side stays, etc). While the 550 is usually black (macho thing), the 2mm can be all sorts of fancy, shiny colors, especially if you don't want to trip over smaller side stays.
If you're interested in 550, you can buy spools of it from Amazon for peanuts. Here's a link: