I think you choose those two complex forms based on what you want to do. For example, I'd played a hacker some in 5e, so when making a technomancer in 6e I didn't want it to just feel like a decker without a deck -- I wanted some funky matrix magic! So I took Pupeteer and (name might be matrix veil? The one that lets you make matrix illusions). I've only had the techno in one high stress encounter so far, but they made a matrix illusion of an additional attack drone coming in at a different entrance to throw off the matrix intell of the opposing side, and used Pupeteer to pop open a door without having to worry about full-on hacking it.
I'm sure that there are plenty of good choices based on what it is that you want to do. Once you know your objective, then find the best complex forms for your objective.