Your question has omitted an important element: do you want to shoot colour or b&w?
And it contains a certain contradiction too, you want to make portraits, but in low light only?
To my mind, making portraits involves most of all a certain ability to use the lighting for best effect - you either use the northern window type of light, but then you need static (collaborative) models, or flash one way or another, is indispensable. What counts most, is the possibility to focus accurately and controlling the depth of field helps. This puts an SLR in advantage. If on top of this you want to do it in colour, forget the film altogether.
On the other hand, if you are after a first rate b&w portait, including some ambientation of your subject - i.e. from half body to full body plus part of the room portrait, then a rf is a winning proposition - a Mamiya7 with a 65mm or 80mm lens plus Nikon Coolscan 9000 will probably set you back as much as a Canon 5D with a decent L lens, but the result will be much more rewarding, and even a Bessa R3A with a 75mm Voigtlander lens plus a Coolscan 5000 will be difficult to match in b&w.
The final point is: I have MF cameras, a DSLR and a SLR, but I find myself grabbing a Bessa with a small lens, loaded with XP2, whenever I go out of my home - why?
Because this is the most portable high quality no hassle combo I have, and as the adage goes, the best camera is the one you have next to you when the opportunity comes, so for me now this is the best camera.