Hi Redmondyu,
What kind of dim light are you talking about? If your subjects are inanimate, any half decent camera and lens can shoot fine in dim light if you put it on a good steady tripod, so that you can shoot long exposures.
If your subjects are moving/living then you need to address the problem with equipment. Both cameras you mention are about equal for low light work. If you want a body that is appreciably better in low light, you would have to go full frame, like 6d.
The biggest thing for you would be bright fast lenses. A wide aperture lens is one that opens wide to let light in. They call them fast lenses because when they open that wide it lets you use a faster shutter speed than you could have used with a narrower aperture. The best bang for the buck in wide aperture lenses are the fixed-length (non-zooming) lenses, also called "primes". They are both sharper and brighter than zooming lenses costing 2 or 3 times what they cost.
The kit lenses you get in bundled packages will NOT be good in low light. When deciding what body to get, be sure to leave enough money in your budget for a wide aperture lens. A prime like a 50mm f/1.4 lets literally 4 times more light into the camera (thus lets you shoot in 1/4th the light) compared to the kit lenses.
To make your budget stretch enough to get the equipment you need, including a bright prime, consider getting the lesser priced body, or consider getting the body without the included discounted kit lenses. Or both.
Scott
Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites
Why do so many people say "FER-tographer"? Do they take "fertographs"?