INTJs are always changing the problem space, either by redefining the problem, or understanding that the problem is something else entirely. This goes along with what Uumlau wrote about staying current with changing reality.
INTP then might look for an all-encompassing solution that will covere every possibility. The problem is it might require too much time or money, or be too cumbersome ever to be implemented. INTJ will realize that a useful solution need not include all cases. We are contingency planners, so our solutions usually do include some built-in flexibility (we plan a picnic, and specify a rain date), but we probably are not going to cover all the possibilities that the INTP ideal solution will. We will come up with something that can actually be used, though.
In asking whether INTP or INTJ solutions are better, there is a third route: combining them. I work with a number of INTPs, and my SO is one as well. Sometimes it feels as though when we work together well, there is nothing we can't accomplish.