T1 is a testable, scientific theory to solve problem P. T2 is a significantly less testable theory to solve P. In Popper's view, barring some important other consideration, if both T1 and T2 are non-refuted then we must prefer T1 and say it's better.
But T1 might not be better. You could easily choose T1 so it's false and T2 so it's true as best we know today, without contradicting the situation description.
You can assert that T1 is better, as far as we know, given the current state of knowledge. But is it? Where is the argument that it is? This looks to me like both explanationless philosophy and positive philosophy (T1 is supported by its testability, and T2 isn't). T2 is losing out without any criticism of it.
What we should do is not say T1 is better, but say: T2 needs to be testable to be a viable theory because X. X can be a generic reason such as scientific theories should be testable and P is a scientific problem. Once we say this, we are now making a criticial argument: we're criticizing T2. This offers T2 the chance to defend itself, which never came up in the original analysis.
It's now up to T2 to offer a reason that it doesn't need to be more testable, or actually is more testable. T2 can criticize the criticism of it, or be refuted. (BTW if T2 didn't already contain this reason, and it has to be invented, then T2 is refuted and T2b is now standing, where T2b consists of the content of T2 plus the new content that criticizes this criticism of T2.)
Then if the testibility criticism is criticized, it can either be refuted or be ammended to include a criticism of that criticism. And so on. This approach takes seriously the idea that we only learn from criticism. That makes sense because criticisms are error-correcting statements: they explain a flaw in something, which helps us avoid a mistake.