Does the camera really matter?
The NEW-era Sony-designed EXMOR generation and newer sensors are ISO invariant, allowing the user to recover and brighten-up exposures made at shutter speeds that are four or even five full EV values "too dark"...meaning that in dreadful light, the new-era Nikon and Sony and Pentax cameras give you a simply huge advantage. Huge. The kind of advantage where a BLACK frame can be "lifted" (in other words, brightened-up) in post-production software, to make a usable picture. This is the main difference between the old-tech Canon sensors, and the newest, state-of-the-art sensors used in Sony,Nikon,and Pentax d-slr cameras.
Yes, the camera does matter if you are still using an old-tech sensor, which is what Canon put in its Rebel series bodies for something like nine years in a row...the same, basic 18-MP sensor that has since been bypassed by newer-tech sensors in "other" types of cameras. Canon's newer 6D and 5D Mark III full-frame cameras have better sensors, but are still unable to do the 4- and 5-EV "lifts" of dark exposures that the Sony,Nikon, and Pentax cameras can easily do. This is what a truly "ISO-invariant" sensor can do...and this is the one area where the camera truly does offer a HUGE advantage, in the field, and in the studio.
In lower light conditions, it's often just a better idea to bring in ancillary lighting gear: a reflector is a HUGE boost in many situations, if one can be used. Electronic flash is a HUGE boost, many,many times. Even a little, tiny 1/8-power flash pop can make a difference indoors or outdoors, when the light is poor!
YES, the user and his or her skills are important, but the reality now is that there _does exist_ an entire group of cameras that have exposure and exposure-recovery capabilities, in which a severely under-exposed, pretty much black image, shot in raw capture mode, can be "lifted up", and a usable-to-good picture made, by deliberately under-exposing (or when accidentally under-exposing!) and then using software to brighten up the final picture. So, yes, the camera does matter.
I started shooting with the Nikon D1, in 2001; the Nikon D610 or D800 are truly exceptional cameras, compared to that old dinosaur. Compared to my Canon 20D from 2006, or my Canon 5D Classic, the new-era Nikon D3x or D610 or D800 are __vastly better__ imagers, across the entire spectrum of ISO levels, and lighting conditions.