Both metrics, along with ACWP are necessary to measure real project performance. BCWS is the time-phased budget for the work you planned to do during a given period in the project. ACWP is the actual cost of whatever work you performed during the period. BCWP is the earned value of the work you actually completed during that period. The cost variance of CV is the BCWP-ACWP and if that number is negative, then you spent more than you planned for that period and you experienced unfavorable cost performance. The schedule variance SV is the BCWP-BCWS, and if that number is negative, then you accomplished less than you'd planned for that period and you experienced unfavorable schedule performance. You need to monitor bot the cost and schedule performance of your project to know how you're really doing.