CREST is an Excel model that was originally developed in 2011 and has not been modified since then.
SAM is a desktop application that is currently being maintained and updated.
CREST's main output is the levelized cost of energy, which SAM also calculates. The financial models in SAM and CREST are both based on annual cash flows and use a similar approach. CREST's implementation in Excel may be easier to work with if you are accustomed to financial modeling in Excel. (SAM can generate Excel versions of its financial models, but these are designed to illustrate how the financial models work -- they are not fully functional Excel models.)
SAM has a more detailed model of the system so it generates detailed outputs of the system's performance that are not available from CREST. CREST's annual production estimate is based on a simple equation using nameplate capacity and capacity factor inputs, while SAM's is based on a detailed hourly or subhouly simulation.