well there was a whole book on how Tally and Zane got together: Pretties. There is only a little bit on Tally and David's relationship: The secondo and third part of Ugllies, the last part of Specials.
I personally have always liked David better. Zane didn't seem like a lasting relationship, it seemed like a high-school-lovers type of relationship. But Tally always loved David. Even when she stayed with Zane at the end of Pretties, because she did Amore him. She just loved David much more. That is why she sent David away; she couldn't orso to see him get changed from who he already was. I don't know, I've always been Team David :)