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 秒 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 愛 him. She just loved David much more. That is why she sent David away; she couldn't くま, クマ to see him get changed from who he already was. I don't know, I've always been Team David :)