I have been using a Sony NEX-5R for around an year, and I have played with a friend's Nikon D5300 for a few days. That makes me not the most qualified person to answer your question — that would be someone who has used both an SLR and a mirrorless camera for years. But almost everything I say below is verifiable, so you don't have to take my word for it.
OVF: One important factor in the choice between SLRs and mirrorless cameras is whether you care about an OVF. An OVF is generally preferred to an EVF or an LCD, but since you mentioned low-light photography, I find that an OVF doesn't help for low-light photography — the view is dark, and I can't see clearly what I'm shooting. Whereas, with the LCD or (I'm assuming) EVF, the image is digitally magnified, so you see what you're shooting. In my case, I care more about night photography than anything else, so the right decision is to avoid an OVF and therefore avoid an SLR. Depending on how important night photography is to you, you may find this applicable, or not, to you.
Controls: SLRs have a lot of them, but not low-end mirrorless cameras. So, if you care about having lots of manual controls, and you're buying a mirrorless cameras, you should probably buy one near the top of its respective lineup (A6000 for Sony E-mount, and E-M1 for Olympus m4/3). An SLR will probably have more controls than even the A6000.
Size and weight are a big difference. My NEX-5R is roughly twice as high as an SD card. It's one thing to read the specs, but, concretely, SLRs feel bulky and heavy. When I'm going to be out for hours, with other gears such as multiple lenses, a tripod and so on, that too in an environment that's likely to be too hot or too cold for me, or I'm likely to be hungry or thirsty or tired, I'd much rather carry a mirrorless camera. Again, only you can decide how much this factor matters to you.
Battery Life is worse with mirrorless cameras than with SLRs. I find this to be irritating, to constantly keep my camera charged because otherwise if I just grab it and go somewhere, I might run out of juice midway through my shoot. It adds to the hassle of owning and using a mirrorless camera, when you have to constantly keep worrying about charge, and not have the confidence that the camera will have enough juice for a full day of heavy shooting.
Autofocus: I use a Sony NEX-5R, and I played with a friend's Nikon D5300, and I find that autofocus performance on the Nikon under low-light is not reliable compared to the Sony. It would often fail to autofocus and not let me take the photo. Note: I'm not talking about the speed of autofocus, but the reliability — what fraction of shots correctly autofocus. Again, only you can decide how important this is to you.
Lens selection is good with Nikon and Canon SLRs (I don't know about Pentax and I don't want to talk about what I don't know, so I'm qualifying this statement by saying Canon and Nikon SLRs), and with micro four-thirds, but not so good with other mirrorless systems like Sony (or, even worse, Samsung or Fujifilm).
To take the E-mount as an example, you have all the types of lenses you'd need: F1.8 primes at a couple of focal lengths, a cheap ($200) but sharp and good F2.8 prime, an ultra-wide-angle zoom, a 3x zoom kit lens, a high-quality constant-aperture F4 18-105 zoom and a superzoom. Or, if you want to avoid the superzoom, you can buy two lenses covering the same range of focal lengths. There's also a pancake lens and, I think, a portrait and a macro lens. So, all the various kinds of lenses you'd want are there.
But the selection is limited compared to Canon or Nikon SLRs:
- I wanted to buy an 18-135 lens, but it doesn't exist for the E-mount.
- The E-mount stops at 210mm, while Nikon goes up to 300mm or so, at an affordable price.
- If you want to buy a superzoom, with Sony, you have only 18-200 options, while Nikon gives you an 18-200 and an 18-300.
- There's no constant-aperture F2.8 zoom.
So, if you're buying a mirrorless camera, and you're buying something other than m4/3, you'll have to make some compromises — you may not get exactly what you want.
Affordability of lenses: I know only about Sony E-mount, so let me describe that as an example. E-mount lenses are moderately priced. You have a F2.8 19mm for $200, a F1.8 35mm for $450, a high-quality 18-105 for $650, and so on. Again, these are indicative prices, to give you an idea.
Fujifilm, btw, is the outlier among mirrorless cameras — extremely expensive lenses ($600 - $1000), but supposedly high-quality. So, if affordability of lenses matters to you, avoid Fujifilm.
Image quality is generally considered to be comparable between mirrorless cameras and SLRs, certainly ones with the same sensor size. As for megapixels, since your question mentioned it, Sony has a 24MP camera, and the E-M1 is 16MP.
Micro four-thirds: If low light photography is important to you, keep in mind that you have a disadvantage with smaller micro-four thirds cameras. I shoot low-light more than anything else, as I said above, so for me the right choice was to go with an APS-C camera, whether mirrorless or SLR. This may not apply to you if you do night photography only once in a while. For example, Dxomark found that the Sony A6000 and the Nikon D7100 give good results up to around ISO 1300, while the top of the line m4/3 cameras, the E-M1 and the Panasonic GH4, top out at around ISO 800.
In summary, I agree with the other posters that you can't go wrong, but I hope this has given you a good idea of the pros and cons of mirrorless cameras vs SLRs.