I have not used the Sigma you refer to, but I have extensively used (and have owned for some time) the Nikon you refer to. I've shot it on both the D3200 and D7200. Usually the only lens I carry... and for a 3 week tour of Australia, it was pretty much the only lens that ever made it out of my backpack.

I do not have a full-frame camera, which is why I am looking at the 35mm focal length and not 50mm.

The DX "crop-factor" makes the 35mm DX behave roughly as a 50mm on 35 film.

(a) would fit a full frame Nikon if I decide to upgrade at some point, which is certainly possible a few years from now

The DX will fit a Nikon FX. Assuming it's anything less than 8 years old, the FX camera will automatically fall into "crop" mode and shoot using a reduced portion of the sensor. That is to say, it'll work - but behaving as if it were a DX body (with less than half the resolution of the full sensor). You'll get that "50mm-ish" equivalence.

The Sigma 35mm will behave as a 35mm on an FX body. You'll get full frame use, and it'll be significantly wider than it behaved on the DX.

(b) has a wider max aperture

Shooting at 1.4 is going to give you seriously shallow focus. You can see how shallow the 1.8 DX goes here: http://www.kenrockwell.com/nikon/35mm-f18.htm

I've found I rarely get down to f1.8, nearer 2.0 or 2.2 is vastly more common. Obviously it depends what you're trying to do...

[Upgrading to an FX] which is certainly possible a few years from now

The DX sensors are pretty awesome. They're getting better every day too. Getting up to an FX (and I have shot on one) is not only a huge leap in price but also a huge leap in weight... and ultimately (I believe) does not automatically add to the photos. The photographer makes the photos, not the camera.

DX bodies with the f1.8 35mm lens you're looking at are light, robust, and will do a lot. Even in the dark... even hand held. (There's no VR [vibration reduction] on that lens FYI.)

I've uploaded some personal photos shot on mine for your reference to flickr: https://flic.kr/s/aHskZb57Kg. Flickr pulls the EXIF data, so you should be able to see what settings I used. I've specifically included some "bad" examples where I feel the f1.8 depth of focus was too shallow.

I have hundreds of example shots - I've tried to pick a variety of situations. Leave me a comment if you want a specific example, and I'll happily have a hunt.

Hopefully someone else can comment on the Sigma!