Tifa
"I wonder what Aerith felt... when she was on that altar...?"
Cloud
"I'm sure she wanted to give her life for the planet..." ~ FFVII
Implying that Tifa, knows Aerith more than Cloud does is beyond ludicrous; Here we have Cloud and Aerith who immediately built their own two person world, they immediately became close, Cloud was willing to go to Shin-Ra building and save her even though they knew each other for only a few days at the time.
Cloud
"...Aerith. Aerith has already prayed for Holy."
"...After I gave the Black Materia to Sephiroth...... Aerith's
words came into my dreams..."
"She said, she was the only one who could stop Sephiroth......
And to do that, there was a secret here..."
"That was Holy...... That's why, she had the White Materia.
Aerith knew about here... and what she had to do."
"Aerith has left us great hope. But, it cost her her life... her
future..."
"I'm sorry... Aerith. I should have figured this out sooner."
"...You left us without saying a word...... It was all so sudden,
so I couldn't think..."
"That's why it took so long for me to find out... But, Aerith...
I understand now."
"Aerith... I'll do the rest."~ FFVII
Trying to use Tifa's next comment to Cloud to override Clouds words which began puzzling the pieces that Aerith left behind for him together.
Aerith opened up to Cloud on numerous occasions, At the Cosmo Canyon, when Tseng died, when she entered his dreams and decided to say goodbye to him of all her friends.
She also had good feelings towards him. At first, she thought he somehow had some similarities to her first love. Even so, his looks, voice and personality weren't similar and he also made her think of him as a mysterious person... But it soon didn't matter. She loved him much more than her first love. Cloud was her hero and he couldn’t get away from danger. She saw him as someone full of confidence, cool and had the impression that he would disappear in an instant if she took her eyes off him. She wanted to stay by his side forever if she could. She really wanted to.~ Maiden of The Planet
Why couldn't she stay with him forever? She wanted to? Its almost as though she knew something was standing in her way of doing so.
When she left her companions and headed for the Forgotten City, Cloud's heart was like an egg that was on the verge of cracking open. It wasn’t going to crack open like the way an egg hatched but, as if only the yolk was going to seep out of it. It was as if his mind was going to shatter. She wanted to comfort him. If she wasn't the last survivor of the Cetra she probably would have done so without a doubt.~ Maiden of The Planet
She wanted to, she could have if she didn't think or know she would die. But here her words are contradicting other wise, Aerith who spoke with the planet, and her dead mother who told her she would find her promised land-henceforth; dying.
That was where Aerith hesitated. Will she let all humans die or was she going to avoid such a disaster in exchange for her life... But she never did think about it and was already prepared. When she did hesitant about leaving Cloud in sorrow, she would think about how it wouldn't save her companions or the people of the world. She had already made up her mind. There was no other choice. It was all for Cloud too. ~ Maiden of The Planet
Again she had to choose between death as a sacrifice, or to stay with her friends and let the world go to oblivion. She even wanted to stay behind with Cloud but fearlessly faced death despite her desire.