Dear Muhanna, I would use Optical density for sure and to make a good growth curve, you should take the same sample volume (ex. 100ul) every one hour (triplicate) for at least 16 hours or until no further growth is detected. But before, make sure that your culture is homogenized (by steering or vortex). 

Unfortunately, the answer depends on the purpose of your growth curve.

If you are looking for a certain cell density before manipulating the culture (adding IPTG, assuming start of sporulation/antibiotic production, etc), usually researchers use optical density (since it's easier). I agree with Wael, particularly with streptomycetes it's important to vortex vigorously to break up any mycelia that can result in clumping and inaccurate values.

If you are looking for viability (antimicrobial effects, etc), you should probably use dilution plating to determine CFU/mL. This is because any particulate will block the light path in OD measurements so dead cells /cell fragments contribute to your OD. Again, vigorous vortexing is critical so that "CFU" are as close to "individual bacterial cells" as possible - otherwise a clump of mycelia can present as a single CFU. Dilute Tween (0.002%) may help break up the clumps but some streptomycetes are sensitive to it.

Cell mass is a crude measurement and generally is not recommended. Hope that helps! Good luck!