According to my experience ISTP's are more into this day (Se), this moment and INTP's live in the future (Ne). ISTP's are quite aware and alert of things happening around them (Se), INTP's are not. ISTP's (T+S) are less interested in abstract theories than INTP's (T+N). ISTP's are more in tune with their bodies (Se) than INTP's. ISTP's are much more prone to notice details than INTP's. There might not be any difference in logical thinking (both probably get pretty high scores in intelligence tests that measure logical thinking) because both have dominant Ti, but the interest is usually divided to the concrete with ISTP's and abstract with INTP's (not always of course). Both types can be pretty hard to know.