In need for high-speed data transmission over wireless channels, multiple-input multiple-output (MIMO) technology has been suggested as it exhibits significant improvement in channel capacity. In practice, adaptive modulation scheme can be applied in different MIMO algorithms to further improve the system efficiency. In this paper, we derive closed-form expressions for the spectral efficiency of MIMO system utilizing two algorithms: Orthogonal Space-Time Block Coding (OSTBC) and spatial multiplexing with zero-forcing (ZF) receiver. Furthermore, a low-complexity adaptive scheme that switches between OSTBC and ZF to achieve even a higher spectral efficiency is suggested based on the closed form expressions.