Faith and Riley are not characters that people tend to see as similar, or as having similar relationships with Buffy. On the surface, they couldn't be more different: Faith is the bad girl from the wrong side of the tracks, Riley is the privileged good boy.
Even if you dig deeper, they are very different. Riley is an uncomplicated guy who didn't have his worldview seriously questioned until he was in his twenties. Faith probably isn't quite as complicated as Buffy - but then again, she's never been a protagonist. No doubt, she gives Riley a run for his money in the complications department. Faith's problem is not that her worldview has never been questioned - it's that she doesn't really have one. I remember when I first watched Consequences and then Enemies being confused as to how the slayer of Consequences, who argued that she and Buffy could - and should - do what they wanted because slayers were superior, had become the girl of Enemies, who was motivated more by an intense jealousy of Buffy. But this is who Faith is at this point in her life. Her philosophy of slayer superiority in Consequences is a passing rationalization. It may well have had a deeper effect on Buffy than on Faith.
The one consistent worldview we see from pre-prison Faith is that no one can be trusted. Even that philosophy, however, is not one she actually lives by. She trusts Gwen Post. She trusts the mayor - in part, I think, because he is so open about using her and grooming her to be a killer. There are no surprises there. That's what love is, in Faith's world. Using and betrayal.
And, to a certain extent, she even trusts Buffy. She has no solid worldview. In Faith's world, nothing's in control, and nothing makes sense.
I wonder what Buffy and Faith's relationship would have been like had they met in S1, or early S2. Before Surprise/Innocent changed Buffy forever. In Faith, Hope, and Trick, Buffy is able to see herself in Faith, the traumatized girl who chooses to run away. She is able to reach out because she understands.
But she never tells Faith that. She refuses to discuss Angel with Faith. Just about everything Faith learns about Angel comes from someone besides Buffy. One of the reasons Faith doesn't want to open up to Buffy is that, from her point of view, Buffy doesn't open up to her. Buffy doesn't love her.
Riley does find out about Angel from Buffy, but it's after they've been together for awhile. And she leaves out the little detail as to what, exactly, triggered Angel's turn to evil. Riley was attracted to Buffy in part because she was a mystery - but the more she discovers of her past, the less certain he feels of her role in her life.
Buffy, by contrast, became attracted to Riley because she thought he was "dependable". The reality is that he wasn't. Oh, he had no dark secrets, he was a step up from Angel in that regard, but stability isn't measured by how you act when everything is under control. It's how you deal when everything falls apart. Riley, when Buffy met him, had never been tested. He's more stable than Faith - but then again, your average earthquake is more stable than pre-prison Faith. But he's not stable enough to handle Buffy's life.
With so little information about Angel, Riley and Faith reached the same conclusion: Buffy's relationship with Angel is an attraction to vampires as a group, an attraction to violence in general. Riley wondered what kind of hold creatures like Angel and Dracula had over her. Faith told Buffy that Buffy was probably still into Angel when he was killing Jenny Calendar and Willow's fish.
This shared perception of Buffy means very different things to them. Riley was a "good boy" who came lose his super-powers. Faith was a slayer like Buffy, who could never lose her super powers, and was rapidly heading down the road to evil. Riley's attempt to make himself "dark" by letting vampires bite him was pathetic, and he ended up leaving Buffy because he felt he couldn't keep up with her. Faith's attempts to drag Buffy into the darkness with her met with mixed success, and she was never able to get Buffy to love her the way she wanted her to. By the time she truly came to understand why Buffy didn't exploit her powers, her friendship with Buffy was nearly irreparable.
Of course, Faith and Riley did meet, sort of. And one of the key moments of Faith's redemption arc was when the faithless, directionless bad slayer encountered the wholesome, uncomplicated good boy who had just had his entire worldview fall to smithereens.
Faith-in-Buffy's-body comes to tempt Riley with her badness, and instead he tempts her with his goodness. I think it's safe to say that Faith had never had a loving sexual encounter in her life. Riley's as turned on by her as she expects, but he doesn't want her to be a "bad girl". He doesn't want to "hurt" her. He just wants her to be Buffy, and he just wants to be Riley. In Consequences, Faith taunted Buffy about the "sex" and "danger" of "screwing vampires", as if that's what enjoyable sex was. Safe words are for wusses, after all. But when she sleeps with Riley in Buffy's body, she has an entirely different experience of sex.
When Riley tells Faith-as-Buffy that he loves her, her first reaction is to assume he wants something from Buffy. But she knows he does not. This is what Buffy has that Faith wanted but could never articulate, because she never really knew it existed. Angel was a stalkerish mass-murderer, so his feelings for Buffy could never be a simple love. Faith could easily paint Buffy's relationship with Angel as Buffy being turned on by danger. Any noble feelings Buffy and Angel might feel for each other were tarnished by the horror he had perpetuated on Buffy and her social circle.
But Riley was different. Riley's feelings of love were simple and uncomplicated. You be Buffy, I'll be Riley. It was through Riley's love that Faith started to truly see that there were good people out there, that trust wasn't just a way to set yourself up for betrayal, that there were people out there who weren't just interested in using people. That love didn't have to be a messy, dirty, complicated emotion.
And there's deep irony in the fact that the very personality traits that helped Faith to see what it was like to be Buffy, were the same traits that led Riley to conclude that he could never be what Buffy wanted.