I would say that if you test as INTP or INTJ, then likely you would test as one or the other on the official MBTI, if you are comfortable with the general descriptions of the two. If you're comfortable seeing yourself as INTX and letting yourself discover eventually which one you are, then let it go.
But if you want to take the best test there is, and have someone to help you with follow up, take the official MBTI, from a certified MBTI adminstrater. You are far more likely to not only find your true type, but understand what it all means.
Weighing the cost of the MBTI with how important it is to you to know your exact type is up to you. I think the majority of people (like 60 to 70% maybe) can be comfortable with the results of online tests. But if you find you are having difficulties really deciding and are bothered by it, then I'd take the plung with the official MBTI test.
Personally, I took the test first from Keirsey's short test at the beginning of his book Please Understand Me. I tested INFP, and it fit. No problems. I'm just rather decidedly INFP. Last time I took the official test was in a week long MBTI training program about 15 years ago and I score about 100% for each function. So, I can no longer take a test objectively because I know what each question is trying to answer. I'm sure the same is true for a lot of the "oldies" around here.