One of the tests that we can apply to our own selves when we sit for concentration or meditation, as to whether we are well prepared for it or not, is to see how we feel when we sit for concentration. These questions each aspirant may put to himself, and the answers that come would let him know where he stands. Choosing the Point of Concentration Teachers of Yoga have hundreds of things to say about concentration. Each teacher will propagate his own technique - whatever he has studied, or whatever he has heard, or whatever he himself is doing as a practice. All these are methods, are valid techniques.
Any method is good enough, provided it is resorted to in right earnest.
It will be difficult to decide this at once. Because, both alternatives will look all right, and yet the student will be oscillating between the two alternatives. Even supposing he comes to a decision as to whether it should be the outside or the inside, he will not know how to conceive of it. It is a foolish statement. It is impossible not to think anything unless one is sleeping or one is in a state of supernal absorption in a high state of consciousness.