The ISP acceptance rate is quoted to be around 30%, I believe, but no one that I know has encountered a student who attends Northwestern and was rejected from the program. In any case, ISP tends to prioritize an inclination toward math and a well-rounded interest in the sciences, so if you've pursued extracurricular activities in math and have generally done well, your chances are probably pretty good. I wouldn't worry =).
I'm not terribly well-informed about MMSS, but it seems to operate under similar principles as ISP, except with a humanities focus instead of a science one. I'd figure your decision could be made based on this: are you more interested in political science, econ, and business type-fields, or in physics/chemistry/biology type ones? You could research some of the alumni of the programs as well, and see which group is following the career path you'd prefer.
As for ISP, physics and EE are closely related on some subjects, and computer science and natural science interface well in theoretical studies. So there're some possible applications there, where a science-focused program could benefit you. Either degree would require more classes taken in the field to double major in it than would an additional degree in phyx/math/chem/bio. It's still very possible, though. To help graduating in four years, it might benefit you to transfer schools to McCormick, since both EE and CS are degree programs there. You could then still be involved in ISP, but not be required to have foreign language credit and take full advantage of any AP credits you have to this point.
Serithsky, I'm curious about something. This is based strictly on what I've seen on the NU web site.
NU appears to strive to be a leader in interdisciplinary study. Case in point are the ISP and MMSS programs, both of which have been around for about 30 years or so. It appears that MMSS does a better job of "marketing" itself, though.
MMSS web site site info on it includes links to past senior theses, pictures of current students in the program, entry allowed during freshman and sophomore years, etc. It's also a larger program (maybe that has something to do with it), perhaps because it's more popular.
ISP is smaller, has less info available on the web about it, and "surprises" STEM applicants to NU when they receive info on the program after they've applied to NU.
Is all this by design or by default for ISP? In other words, do they feel nothing more is needed to grab the kind of students they desire? Or is everyone just so busy doing their thing in the lab that no one has paused to ponder this?
ETA: ISP web site info does show pictures of graduates from ISP, just not current students in the program.
paragonD6, that's about what my S had in his application. He was a little concerned of some overlap with items in his regular NU app, but as that also dealt with his desire for interdisciplinary science study, I didn't see a problem ... and neither, apparently, did ISP.
I just had lunch with another parent who's son has applied to NU McCormick. He, too, got the informational materials about ISP. So it appears ISP casts a wide net, as it's in Weinberg but engineering is in McCormick. Perhaps NU is so accepting of the idea of interdisciplinary study that these lines are blurred. However, they are different colleges with different distributions for graduation requirements.
This all becomes more interesting to me, as my ISP son has just recently expressed enough interest in engineering that he's going to speak to his ISP advisor about it. He claims getting into Weinberg isn't a big deal for an ISPer, and that there are some students in engineering and ISP. I call that being very ambitious!
There are/have been ISPers who've done the ISP/Comp Sci combo.
I'm not in the field, so no comments here as to whether grad school is advisable. It probably depends on a lots of variables you won't even be aware of until after you've been in the program for awhile. That said, many ISPers do go on to grad/prof school. Check with the ISP program itself to see whether these students do it directly or stop off and get some job experience first.