Short version of my question: How can I compare cameras with different optical zooms and different megapixels (more megapixels can allow cropping for a fake zoom effect) in order to tell which camera can achieve an equivalent of a 5MP image shot with full zoom (x10 --> 360mm) on my SX120 IS?
Long version: I own a Canon SX120 IS (1/2.5", 10MP, 36–360mm, F2.8–4.3). Lately its image quality in medium to low light has become insufficient for me. As I understand, this can be resolved by getting a camera with a larger sensor, 1" for example.
I still want a compact camera (including lense, without having to carry other lenses) but won't get a mirror-less camera since I don't want to give up the x10 zoom ability. I am, however, willing to compromise a little bit in that area.
It seems as if the Panasonic Lumix DMC-ZS100 (1", 20MP, 25–250mm, F2.8–5.9) is almost perfect for me. However, it costs much more than I was planning to pay. My other options are Sony CyberShot DSC-RX100 (1", 20MP, 28–100mm, F1.8–4.9) and Sony CyberShot DSC-RX100 II (1", 20MP, 28–100mm, F1.8–4.9). The problem is zooming with both RX100 cameras isn't as good as with my Canon. They do have a feature called Smart Zoom that crops the image, so instead of a 20MP image you get, 5MP or 10MP. That should be enough for me, and it's not ordinary digital zoom as the details are maintained. Yet, I don't know how it compares against my SX120 IS. Sony's article states the Smart Zoom cropped to 5MP is equivalent to X7.2 zoom, but that x7.2 is what it would be like if that same camera actually had that zooming ability.
How can I compare the zooming ability using Smart Zoom to my own SX120 IS?