In this paper, a comparison study between gyro-based and gyroless approaches for spacecraft attitude estimation is presented. Due to its vulnerability to the model errors, the gyroless approach has not been widely focused on and there are only few comparison studies available. However, this conventional wisdom might not directly apply to CubeSat attitude estimation, where noisy MEMS gyro is usually implemented. Although the noise density can be improved by low-pass filtering, it sacrifices the bandwidth so that it can induce a discretization error when spacecraft rotates in high speed. This paper outlines expected pros and cons of gyroless attitude estimation with respect to cost and miniaturization, rotational agility, and model errors. Additionally, linearized system models for both of the attitude estimation methods are formulated and a simple guideline for tuning process noise against the model errors is proposed. Numerical results for a realistic earth observation scenario are presented to quantitatively compare the benefits and drawbacks of each attitude estimation method.