If the university accepts both ISFAA and CSS, will it lower my chance to get a need-based financial aid if I send the ISFAA form?Or are they equivalent?
They should be the same. You only consideration is that some school only accept CSS, while some accept both, while some accept ISFAA only (like Uchicago). The CSS has an advantage is that you can upload the documents on the IDOC once only, while for the ISFAA you need to upload the documents to each school specifically, I think…
Overall, I would go for the ISFAA as it is basically free.
take note that the questions are a little bit different for the two. E.g. on the ISFAA they specifically ask you to fill in educational expenses and health insurance, but you have to remember to fill those in under “other” on the CSS.