You were dating Tohsaka-san. I'm not interested in you that way, even if I wasn't seeing someone. That inherently makes my feelings about you different than hers.

[Duh?]

I agreed to become your master because as you said, I'm a magically inclined person you can trust to release your contract when Tohsaka-san returns.

I'm doing this because I like you as a friend while she most likely did it because she either wanted your services or to become closer to you. I don't know her well enough to know which, but that is different than my feelings on our relationship.