Short answer? If you don't know what lenses you want, go Nikon or Canon. They have full-frame bodies in their lineups. They have the largest current lens lineups. For full scope of future upgrading, you can't go wrong with either one. Other brands have great bodies, but their current lens lineups don't compare, and you can find yourself locked out of choices you may want in the future. (If otoh, you know
exactly what you want, and another brand has the lens lineup that fulfills those needs, then go for it.)
You cannot interchange lenses with different mount systems (without losing autofocus and autoexposure and EXIF functionality). So if you go Nikon, you'll be buying Nikon lenses or Nikon-mount lenses from Sigma/Tamron. If you buy Canon, you cannot use Nikon or Pentax or Olympus lenses. So, the decision you make locks you in. Nikon & Canon are therefore going to be your "safest" choices, especially if you don't know what lenses you want.
Be aware that jumping from a P&S to a SLR is not a simple straightfoward "upgrade", like moving from one P&S to another. There's a very steep learning curve. And you are buying into a camera
system; it's like moving from a swiss army knife to the world of big red tool boxes. Now, at last, you can pound that nail (high iso). But you can also smash your thumb pretty good (out-of-focus), and there ain't no toothpick (face recognition) or tweezers (video) in the box.
The body is just your tool "platform." You're likely to get a new one in five years (like any piece of digital equipment), and the lenses are going to be your long-term purchase. You may eventually spend as much or more on lenses as you did on the body. Be aware that's the kind of cost you're looking at. Some lenses can easily outstrip the cost of the body. The scale of expense of new on-brand lenses is: $100 = "dirt cheap"; $300 = "cheap"; $600 = "moderately expensive", $1000+ = "expensive." "very expensive" starts around $3000.
That doesn't mean you can't start out with the body and the kit lens and a $100 50mm f/1.8.
But you will want more.