My 2 cents, for what it's worth:
The difference between ISFP and ISTP is between dominant-fi and dominant-ti, and between inferior-te and inferior-fe. Here's my understanding of both dichotomies:
Both Fi and Ti are about reductive thinking - finding a root cause and whittling down the understanding of something to what is considered its bare elements. Both dominant-Fi and dominant-Ti types love "clarifying" things, for themselves and others.
- Fi is about emotional reduction - you naturally understand and empathize with root, emotional causes that you identify in people. You naturally like to immerse yourself in these emotions, sometimes to the detriment of your own responsibilities to others. When immature, these types might have trouble reconciling Inferior-Te - they want to take responsibility for things, but they might wave that by focusing on its worst aspects - the way that these arbitrary, external systems categorise and divide people, ignoring their emotional needs in the name of efficiency.
- Ti is about impersonal reduction - you naturally understand the inner mechanics of things, and instinctively see the world in terms of systems. You like to immerse yourself in these systems, sometimes to the detriment of your own responsibilities to others. When immature, these types might have trouble reconciling Inferior-Fe - they want to involve themselves in the lives of others (and are likely to understand themselves in terms of how they relate to others), but they might wave that by focusing on its worst aspects - the way that groups ignore the truth for the sake of tribal relations, clamping down on individual thought and action because it offends the "stupid" sensibilities of others.
Inferior-Te: "Individuals are so divided and discouraged, for such horrid impersonal reasons. I pay attention to how people feel, and that's more important than anything."
Inferior-Fe: "People are so easily manipulated and shallow. I can see how things really work, I couldn't possibly play along. Isn't anyone able to think for themselves?"
I think all IxxPs identify with these ideas (favouring the individual over the group), so if you're having trouble deciding, don't think of it in terms of which one you identify with the most (or indeed like the most) - instead, just try to observe your own thoughts and see which side you tend to land on.