So, the reason you might not be getting a straightforward answer is because it is not such a simple question and many of the parameters required to give a definitive answer are not present.
How long will your tongue be from front of cabin to ball center?
Will it extend to the second xmbr and how far back is that?
A-frame with center member, or just single member?
If a composite tongue, does the center member extend further forward than the 'A', or does it end at the tip of the 'A'?
What size (schedule) of channel (there are many formats with different weights and thicknesses) and which direction were you planning to install it (web up flat or two pieces back to back like an I-beam... or two pieces toe to toe like HSS)?
Maybe some of these questions have been depicted in your prior posts, but it would be helpful if you pull it together with the question.
Personally I prefer box tube for most things. It may be a little more expensive for the same moment of inertia (bending resistance), but it is much easier to terminate when fabricating joints and has more bending resistance in the secondary axis. It is often time more ergonomic and appealing to the eye, too.
Have you seen the
Unified Tongue Strength tutorial in the Design Resources (in the header above^^^)?