Aeris and Tifa are sort of opposites in the sense that Aeris comes across as really gentle and even damsel-ish, but inside is extremely strong and independent, whereas Tifa comes across tough and badarse but inside is more shy and a bit dependent on Cloud (IMO).

If I was a guy I think I'd probably date Tifa, because I have more of the Aeris personality myself (as opposed to Tifa or Yuffie) and I wouldn't want to date someone that I felt was TOO much like me. For a friend probably Aeris because I can relate to her more.

Besides, I think Tifa would be more serious about a relationship than Aeris would. Aeris always seems to be off doing her own thing, and doesn't seem, personality-wise able to settle down with one man just yet. I don't mean to imply that Aeris is slutty, she's just a big flirt.