Background: The administration of a ¹⁶⁶Ho scout dose is available as an alternative to ^99mTc particles for pre-treatment imaging in Selective Internal Radiation Therapy (SIRT). It has been reported that the ¹⁶⁶Ho scout dose may be more accurate for the prediction of microsphere distribution and the associated therapy planning. The aim of the current study is to compare the scintigraphic imaging characteristics of both isotopes, considering the objectives of the pre-treatment imaging using clinically geared phantoms. Methods: Planar and SPECT/CT images were obtained using a NEMA image quality phantom in different phantom setups and another body-shaped phantom with several inserts. The influence of collimator type, count statistics, dead time effects, isotope properties and patient obesity on spatial resolution, contrast recovery and the detectability of small activity accumulations was investigated. Furthermore, the effects of the imaging characteristics on personalized dosimetry are discussed. Results: The images with ^99mTc showed up to 3 mm better spatial resolution, up to two times higher contrast recovery and significantly lower image noise than those with ¹⁶⁶Ho. The contrast-to-noise ratio was up to five times higher for ^99mTc than for ¹⁶⁶Ho. Only when using ^99mTc all activity-filled spheres could be distinguished from the activity-filled background. The measurements mimicking an obese patient resulted in a degraded image quality for both isotopes. Conclusions: Our measurements demonstrate better scintigraphic imaging properties for ^99mTc compared to ¹⁶⁶Ho in terms of spatial resolution, contrast recovery, image noise, and lesion detectability. While the ¹⁶⁶Ho scout dose promises better prediction of the microsphere distribution, it is important to consider the inferior imaging characteristics of ¹⁶⁶Ho, which may affect individualized treatment planning in SIRT.