My closest cousin is an ISFP... I feel oddly protective of her. She's like a "sensitive" me. She admits she takes everything personally, but over time has been better about objectifying it. I often worry that she takes EVERYONE'S feelings into consideration before her own, but she always manages to prove pretty capable. But we're both equally as fun and spontaneous. We used to be partners in crime
We are both laid back. Appreciative of small artistic details that most people wouldn't notice. Seemingly open to lots of things. Can "float around" without a plan.
ISTP is perhaps higher energy, or if not energy, intensity. ISFP is strongly non-confrontational.
Contrast is most obvious under stress. Similar to what StephMC939 says above, they seem to take things personally, even though they aren't from my, (supposedly) more objective view.
They are both SP types so they like seeing immediate impact and rewards for their efforts, they enjoy solving problems (to the contrary I don't think that ISTPs prefer to solve problems that involve people any less than ISFP), and they like their freedom to choose to do or not do. Aside from the obvious in both prefering different dominant functions, I see very little difference except the behind the scenes interaction style may make the ISFP appear more shy than they are and the chart the course interaction style make ISTPs appear more outgoing than they truly are.