In reality, very little. Both of these are in the "normal" range for a DX sensor, and will look like a 42-50mm on a full frame (or traditional 35mm) camera. The difference between 1.4 and 1.8 will likely be lost on all but the most extreme situations.
This is, imho, one of the most difficult lenses to use to make striking photos, and one of the best to learn to use. You're photos will have the field of view which somewhat mimics the human eye's normal focus range. As a result things will seem a lot like you were when you were there (which is to say "boring" for 95% of everyday subjects).
It took me a lot of years to appreciate the (FF equivalent) 28-50mm lens range, and I still tend to grab a shorter (20mm) or longer (85-105mm) range. Still, this is the cheapest way to get available-light, hand held shots because of the aperture. No amount of VR will stop your subjects motion, and even as good as sensors are, the lower the ISO you can shoot the more you'll have to work with in post processing and the smoother and sharper your images will be. Adding grain/noise is easy, taking it away is much harder. Great for indoors or night time with otherwise inadequate light - whether it's a table lamp, a campfire, or a small concert venue. You'll rarely shoot wide open, and even 1.4 is only 2/3 of a stop. Still, the shorter focal length will let you get a bit more in the picture if you're in a tight spot, but it will also increase the depth of field so you won't get quite the same shallow depth of focus you would from a 35mm f1.4.
It's probably a toss up, especially with this level (i.e. non-pro) of glass where the QA may result in more lenses which are out of perfect alignment and you're as likely to get a slightly softer lens as not (IOW - the Nikon may not be a guarantee of a top notch QA at this price point).
Edit: if it were me...I'd buy the 50mm f1.8 for the full frame slr instead. Now, that's a short telephoto on a DX body like the 5100, which you may or may not need. But it would be cheaper, you'd get the "center" of the image circle (the sharpest part). I *love* my 85mm f1.8 on my full frame camera (which is about 56mm on DX) - it's my second favorite lens after my 14-24/f2.8.
I have the 50mm f1.8...the problem is that there is no autofocus! (I have the d5100). I'm just a beginner so it takes me a while to manually focus and I'm never sure if it's exactly right. Also, I can't take any shots where there's moving subjects or it comes out blurry.