I have to say that ISFPs appear much more docile, but can have grotesquely ugly tempers. And man, they can get on a moral high horse also. I know a few who have that "self-denying" attitude sometimes mentioned for Fi-doms - it's like the reverse of the stereotypical SP hedonism. They've become so aware of that tendency in themselves that they go overboard in keeping it in check.
While I am quiet and sometimes seen as "sweet" or just timid, I am more often accused of being aloof than the ISFPs I know. Maybe it's because I am less "present" in the moment, so the ISFPs are literally less detached. I also get accused of being argumentative because I'll take an analytical approach - I want to explore a perspective and the other person sees it as slightly antagonistic. It's not on the NTP level, but there's still that Ne occasional tendency to open up cans of worms inadvertently because you want to explore an issue from many angles. I've seen ISFPs become antagonistic, but it's more a facet of anger than a discussion style taken the wrong way.
I find ISFPs more realistic in an almost yielding to reality sort of way; but I'm not sure that is truly optimistic either. They either have accepted reality and are making the best of it, or they are painfully aware of how far removed it is from their Fi ideals and they are quite depressed or even angry about it. INFPs have cynical moments and moods, but the idealistic aspect is more apparent. It's not optimism either - just a refusal to give up hope for the future; the status quo is not so easily accepted. As a consequence, the ISFP may seem more content with the present, as they focus on working with what exists, and the INFP may be more discontent with the present, focusing on creating ways to change what exists.