For low light photography the two largest considerations are:
- Lens aperture. The larger the opening, the more light can get through. Larger lenses can have larger apertures, and are referred to as "fast" lenses because they allow higher shutter speeds due to the wider apertures.
- Sensor area. The more surface area the sensor has, the more of the light passing through the lens is collected and used for the image.
The mirrorless cameras have been marketed as just as good as DSLRs. Much of the time and for many of the types of shots users moving up from compact point & shoot cameras or bridge cameras take they are. But the most significant area where the mirrorless interchangeable lens cameras (MILC) fall short compared to DSLRs is in low light performance.
There are a few MILCs with APS-C sized sensors, which is the size of the sensors in the Canon Rebel series and the Nikon D3000 and D5000 series. But even when the sensor size is the same, the lens choices available for the MILCs are usually not as fast. If the same lenses that will fit on a DSLR will mount on a manufacturer's MILC offering, the size of the camera body is dwarfed by the size of the lens and the advantage of the smaller size MILC is largely lost. And the sensors in Full Frame cameras such as the Canon 6D or 5D series and the Nikon D300 and D700 series outperform their APS-C counterparts in low light by a significant margin.
If you are determined to use a MILC in low light, find the one with at least an APS-C sized sensor that has the fastest lens options available.