Do the iPhone 7 and 7 plus cameras use the same sensor?
SHORT ANSWER

If it was me and my main interest was in photography, I'd definitely opt for the iPhone 7 Plus over the iPhone 7. Don't get me wrong, both the iPhone 7 and iPhone 7 Plus have great camera systems, but the iPhone 7 Plus has a few features over the iPhone 7 because of its secondary 56mm-equivalent "telephoto" lens system. And, since you're main interest is in low light conditions, you should be aware that Apple markets the iPhone 7 Plus as being the one to use for low-light photography.

LONG ANSWER

Let's start with the specs - below is a summary of the camera specs of each:

iPhone 7 Plus


PRIMARY CAMERA: Dual 12 MP (28mm, f/1.8, OIS and 56mm, f/2.8), phase detection autofocus, 2x optical zoom, quad-LED (dual tone) flash. This features: 1/3" sensor size @ 28mm, 1/3.6" sensor size @ 56mm, geo-tagging, simultaneous 4K video and 8MP image recording, touch focus, face/smile detection, and HDR (photo/panorama). Video recording at:   2160p@30fps, 1080p@30/60/120fps, and 720p@240fps.
SECONDARY CAMERA: 7 MP (32mm, f/2.2), face detection, HDR, and panorama. Video recording at: 1080p@30fps and 720p@240fps.


iPhone 7


PRIMARY CAMERA: 12 MP (28mm, f/1.8, OIS), phase detection autofocus, and quad-LED (dual tone) flash. This features: 1/3" sensor size, geo-tagging, simultaneous 4K video and 8MP image recording, touch focus, face/smile detection, and HDR (photo/panorama). Video recording at: 2160p@30fps, 1080p@30/60/120fps, and 720p@240fps.
SECONDARY CAMERA: 7 MP (32mm, f/2.2), face detection, HDR, and panorama. Video recording at: 1080p@30fps and 720p@240fps.


As I mentioned earlier, Apple pushes the fact that the iPhone 7 Plus is ideally suited to low-light conditions. They even produced a video to highlight this. It's called Midnight and you'll see that the iPhone being used is an iPhone 7 Plus (this is obvious when the user picks his iPhone up at about 5 seconds in).

In summary, with an iPhone 7 Plus you can use the 28mm-equivalent wide-angle lens using Photo mode, zoom up to 10x (digitally), and take snaps with an added depth of field effect using Portrait mode.

If you do a Google search I'm sure you'll find lots of info about the advantages of an iPhone 7 Plus camera system compared to the iPhone 7. I'm sure if you read some reviews you'll find some that tested each in low-light conditions for you to compare.