As a bi-girl, I would say that when I suck cock, I aim to please, and please alone, and I like what I get. But when I eat a pussy, I have this feeling of giving as much as receiving. I'm not sure it makes sense.
Sucking cock awakes the service-slut in me, esp when he's using my mouth hard; while eating pussy is more a shared intimate moment.
I am also a bisexual woman, but unlike you, I suck cock for the same reason I do anything sexual: because I enjoy doing it. If I didn't enjoy the feeling of a cock throbbing in my mouth, jerking my lips wider as jet after jet of semen spurts into my mouth, I wouldn't do it. I also love the flavor of pussy as I usually finger and lick at the same time, enjoying the way a female lover will crush her tasty pussy against my lips as she comes and comes and comes. Of course the best of both worlds is to go down on a woman right after she has been well fucked.