Both are very formal expressions which you would be unlikely to use much in daily conversation, so you would only really need them for writing essays, and even then you could find other ways to express the same idea, so don't worry too much about this. But to me, "inasmuch as [XYZ]" is more likely to suggest that XYZ is true, whereas "insofar as [XYZ]" is less likely to imply that XYZ is necessarily wholly true. "Please help Alex insofar as you can" and "please help Alex inasmuch as you can" would mean the same, because from the context it is clear that the speaker is leaving open the question of how able the addressee is to help Alex, but is implying that the addressee has some ability to do so. "Alex may be helpful insofar as he understands the relevant scientific considerations" does not give a clear understanding of how well Alex understands the relevant scientific considerations, though probably implies that he has at least some understanding. "Alex may be helpful inasmuch as he understands the relevant scientific considerations" is more likely to be taken as positively indicating that Alex understands [at least basically all of] the relevant scientific considerations, so the implications are more positive. That said, at least one online dictionary suggests that "inasmuch as" can sometimes mean "insofar as". So don't get too hung up on the differences.