I feel it depends on the project we are working on if the project is going to be of great value when it will be on the market in it is ok for me to go a little overboard on the cost so the CV to be negative and the Schedule variance(SV) to be positive.
If the schedule variance is negative that is we are behind the schedule and the cost variance is positive that means we are under budget then I feel there will be more pressure on the employees to complete the work as per time and the cost variance will go negative since employees will be working overtime and more materials would be required.
I would prefer a project with negative SV and positive cost Variance(CV) as it would be a challenge to complete the project on time and also be under the budget.