Unless you are trying to get a level to qualify for a contract that specifies what they require, the choice of whether to measure your progress using maturity levels or capability levels is entirely up to you. A few issues may influence your decision.
First, when you “get a level” you are in reality conducting a CMMI appraisal (called a SCAMPI A appraisal for CMMI V1.3 or a Benchmark appraisal for CMMI V2.0). Such an appraisal requires selecting a CMMI model and the scope of the organization to be appraised. In addition, you must determine what results you want from the appraisal.
Capability levels apply to your organization’s process improvement achievement in individual process areas. The four capability levels are numbered 0 through 3. For example, your appraisal results may show that the part of the organization you appraised is capability level 3 for Configuration Management, but capability level 2 for Decision Analysis and Resolution.
Maturity levels apply to your organization’s process improvement achievement in multiple process areas. These levels are a means of improving the processes corresponding to a given set of process areas (i.e., maturity level). The five maturity levels are numbered 1 through 5. The maturity level applies to the scope of the organization that was appraised (e.g., three projects, a department, a division).