m43

OK, so let's say I have a 300mm lens on a Micro43rds body (so 600mm FF equiv) - and the camera shoots at 20megapixels. Now let's say I have a 500mm lens on a FF body and the camera shoots at 45megapixels. Would the FF get a better crop into the image vs the M43 in terms of quality? Seriously...

Found in the "Sword" of the Constellation Orion. 500 light frames and 40 each dark, bias and flat frames stacked in Deep Sky Stacker and post processed in Photoshop Nikon D500, 400mm f/2.8 ED IF AIS Nikkor, 1 sec @ f/2.8 @ ISO 1250

I am looking to buy my first camera and after asking questions in many different forums and watching many videos a new question surfaced . Mirroless or dslr? Do m43 sensors have worse image quality ? APC-S sensors on mirrorless have the same quality as the dslr counterparts? My budget is 400-600...