It's a simple question. How do you know Cloud loves either of them?
In AC, Tifa has what looks like a wedding ring (
a ring, the same token of love given to Rinoa by Squall in VIII. I doubt Square would use same item in the same fucking way, only to have it mean two different things.) They adopted together and they run a business together and they apparently fucking live with each other. The words "I love you" are never stated but the words "HINT HINT" should be heard in your mind when see/read such particulars.
You also have to note Cloud's mental stability when knew Aerith, he wasn't himself. The guy, for numerous reasons, was practically parading around as Zack. Jenova Cells(manipulation by Sephiroth), memories of Zack, and downright shame. He doesn't restore his true personality until he falls into the Lifestream. And that personality is restored with which big chested heroine? Tifa.
After taking all of this into consideration, it can be safe to assume why Aerith even took an interest in Cloud to begin with. During his mental shitstorm, he reminded Aerith of Zack...and we're all aware of their close relationship in Crisis Core.
In my opinion Zack served as big brother/father figure for Cloud, and Tifa served as the big sister/mother figure. HMMMMMM, I wonder why Cloud would call her Mother in AC. I seriously doubt he would confuse his mother with someone he would love romantically, aye? If he did, we might as well just call the fucker Victor Frankenstein.