First, as chocolate said in her comment, a question like this using いい is usually about someone's personal and casual preference: "Which do you like/prefer?". In some contexts, this can be a more objective question (eg, choosing from two business strategies).
Second, the Japanese language doesn't have the simple comparative form (such as better, faster in English), and it has a number of ways to make a comparison without using the comparative form.
Questions like どっちが大きい？, どっちが安い？ or どっちがいい？ are very common to make simple comparisons: "Which is bigger/cheaper/better?" The person who makes such a question is usually thinking the two seem to be equally big/cheap/good. It doesn't imply one is big but the other is absolutely small, one is cheap but the other is very expensive, etc.
But, for example, in a rare situation where the questioner knows one is the cheap one and the other is the absolutely expensive one, we say exactly the same: どっちが安い?
(common; which is cheaper?)
- この2つの指輪は、どちらかは1,000円で、どちらかは1,000,000円です！ どっちが安い？
(uncommon quiz; which one is the cheap one?)