I really hate to bust bubbles in what's a purely hypothetical scenario, but let's actually take a look at Cloud's interactions with Tifa + Aerith in disc one: They're roughly equal, as far as Cloud's treatment of both goes. Cloud's treatment of Aerith on disc one is not "more" affectionate than Tifa--he treats them equally, tries to protect them equally. For every example you have of Cloud showing affection for Aerith, you can match it with Tifa.
So, if Aerith lived, we can assume that this "roughly equal" treatment of each would continue. Then we'd have the lifestream sequence, when it would reveal that Cloud's entire being revolves around Tifa's approval/thoughts of him. Thus we would have Cloud's treatment of Tifa and Aerith being equal, with Tifa now having the additional edge of being his first love that he never got over.
Tifa still would be the undisputed love interest. This would be the equivalent of the "Ragnarok" scene in FF8 (which crushed any dreams of any pairing for Squall other than Rinoa) or the kiss scene of FF10--regardless of whether you were nicer to Yuna, Lulu, or Rikku, Tidus still chooses Yuna. The Lifestream sequence, in a story where Aerith's death didn't happen, would be the same as these other two pivotal scenes, because it holds and serves the same function--cementing the relationship of the final couple before the end of the story. The end result would be the same, regardless of Cloud's affection for Aerith prior to this scene. The other love interests just don't get the guy, and the story moves on. The other love interest just doesn't get the guy, and the story moves on.
And a side note, but Cloud sees protecting Aerith as his responsibility because he "dragged her into a mess she wasn't involved in". (Of course, he always forgets that Aerith inserted herself into the mess quite willingly, but this is about Cloud's mentality rather than Aerith's.) This is the sole reason he is intent on protecting her--because she is an uninvolved party who was thrown into a conflict that had nothing to do with her. It has nothing to do with love/romance/romantic interest--Cloud protects her because he feels responsible for her. This is carried over to her death scene--when Aerith dies, Cloud has failed her. He failed the person he, the entire time, has considered an innocent bystander that he dragged into the whole mess. Had Coud not dragged her into the story, not fallen through her roof she (might) still be alive.
Hardly romantic, all things considering.