Hi. I have own both a ASI178mm (non-cooled version) and a asi290mm-mini. The 178 has a pixel size of 2.4µm and the 290 has a pixel size of 2.9µm. Based on this alone I would assume the asi178mm would offer the more precise guiding. But I also have to assume their are other factors that factor into the most precise guiding. If anyone is a subject matter expert in this matter, please tell me which model to use for guiding for the most precision guiding.
Fyi - I never have issues finding a guide star. I typically guide with a ZWO 60mm guide-scope or a generic 80mm guide-scope. I never use an OAG. My scopes ate an 8" reflector and a 130mm refractor. Focal lengths in the low 800's if that matters.