Our default strategy, and the one I would suggest for your survey, is the Balanced Overlap strategy.
The benefit of Balanced Overlap is it can cause respondents to tell us more information about their preferences. For example, let's say Attribute A is price and the two levels are $199 and $299 and that you used Complete Enumeration, strategy with no overlap in your case. If a respondent is extremely price sensitive, she may ALWAYS choose the less expensive of the two alternatives in your choice tasks and in the end the ONLY thing you learn about her is that she is price sensitive.
Now let's say we built the design using Balanced Overlap. Now some of the choice questions will show both alternatives at the same price, and this will require even the extremely price sensitive respondent to make her decision based on another attribute besides price. In this way we can learn more about respondents' preferences with overlap than we can without.