Well, obviously one is expressed through Se and one through Ne. And now I am going to ramble aimlessly...
I think Se is more focused on the immediate moment, using the 5 senses to channel their imagination into tangible expressions. Even in their altruistic endeavors they seem more hands-on and take-action, with a view to immediate results. They seem to relate their feelings more directly to experience which makes them less... "romantic" shall we say; but there's also less extrapolation to understand foreign feelings. Probably why they don't get the "healer" stereotype INFPs get.
In some ways, I think their Fi may be more difficult to express, because Fi is rather abstract in itself, but Se isn't, and the ISFP has to reconcile the two. The INFP has an abstract outlet to use. I think this encourages a kind of innovation in ISFPs that often leads to so many of them being original & creative artists. I also know ISFPs with "causes" like an NF, but the approach is very different - it's kind of on ground level, content to start small and work up, because Se connects to reality better. However, the ideas can be pretty noble, from my perspective. The whole "DIY" attitude has always seemed very SP to me, but maybe it's the ones I've known.
Ne is more future-oriented, and sees patterns, and forms theories/concepts/ideas with fuel from Fi (or vice verse). So INFPs tend to seek more quixotic means of expression, be it through their moral causes or creating stories to express their ideas through illustration, or maybe creating literal illustrations

. Because the INFP does extrapolate their feelings and apply it metaphorically, it gives the feelings a universal application in the form of ideals. Theory can be trusted over experience because of this, because experience is too limited for the INFP, and it always falls short of this vision of perfection. That's where the idealistic mindset comes in.
INFPs are less action-oriented, but may take action after their ideas have incubated for awhile; and they're more likely to accomplish their objective by simply communicating their ideas, because the idea is an end unto itself. The long term view makes the INFP feel that expression is enough, and again, localized action can feel limiting to someone who is dreaming in such large format. We still deal best in the background, one-on-one, because we are individual-oriented and reaching people as individuals makes sense to us, as opposed to Fe which seems to be more skilled at influencing a group (and I mean that
positively, okay Fe people?).
I also made some comments in a past thread about the differences between an INFP and an ISFP visual artist. Generally, INFPs may tend to be more conceptual & spend more time theorizing how they'll do it than simply doing it. ISFPs sometimes seem to let the expression reveal itself, by becoming absorbed in the physical creative process. That's my shaky theory, and no one really denied it at the time

. I think there is a not a clear line, because I actually do both, but a
tendency can say a lot.