If you're looking for a good mirrorless/rangefinder style camera, I would look into the Lumix series from Panasonic. Although it's not a Nikon/Canon, the reviews speak for themselves.
I personally own a mirrorless (micro four thirds) style camera, and I've owned several DSLR's in the past. The only real difference between a rangefinder style camera and a DSLR is the size and weight difference between the two. Mirrorless cameras are just stripped down versions of DSLRs. The same complexity remains outside of the size and weight differences.
Personally I got tired of lugging around my DSLR, which is why I opted for a mirrorless camera, but never bought into the system. The key differences with buying into a system is the lenses. You just won't find a wide variety of lenses on a mirrorless camera that you can find on a DSLR. Lenses are the bread and butter into picking which system is right for you. Of course Canon and Nikon are top dogs in this field as they have a wide selection of lenses anywhere between wide angle, telephoto, mid-range, macro, fisheye, prime lenses, etc. compared to mirrorless cameras which do have some selection, but just not the same.
So let's say you start off with something simple, but end up wanting more later on. This is normally the case with some people. The cool thing with DSLR's is they're very much affordable now, despite what you may think. Canon/Nikon both have numerous entry level DSLRs that you can stack any of the more expensive lenses on.
So lets say you really want to dive into macro photography, so you buy a good macro lens and slap it on your inexpensive DSLR, you're still going to get great quality images. Most of the time it's the glass on the lens that gives you greater clarity and sharpness in the pictures.
It used to be that camera sensors weren't good enough to control noise at high ISO levels and all that jazz, but now sensors have improved greatly and you will have nothing to worry about. If you're going to invest in an interchangeable lens system, it's best to do so with a company that is going to offer you the right kind of camera equipment suitable for your style of photography.
Aperture, shutter speed, DOF, ISO, etc. are still as important on either system. If you're looking for something portable, go mirrorless. If you're looking to expand your photography, I'd say go with a DSLR.
I personally use my camera phone for everything now and don't really find a need for my mirrorless camera anymore. While the image quality is the same as a DSLR, I find that if I'm just going to post pics online, on Facebook, etc. you don't need all the fancy megapixels, etc.