Can't decide which lens?
I'm in a little bit of a dilemma here. I'm going on a trip soon, and I want a prime lens thats good in low-light to go along with my 18-55 kit lens. However, I also want that prime lens for portraiture use after my trip. I'm trying to choose between Nikons 50mm 1.8g and the 85mm 1.8g. I know the 50mm will be better for the trip, but it's already an included focal length in my 18-55.It's also a decent lens for portraits. I know that the 85mm will be better for portraits than the 50 though, and will allow me to have some more telefoto shots (since I'm using it on a crop sensor, its really 127mm) during my trip. Which should I get! Or If you have another lens suggestion for a Nikon d3200, my budget is about $450-$500
You're right that 50mm is "included" in the focal length of your 18-55mm zoom but you're missing the point of the 50mm f1.8 lens - its 2 stops faster than f3.5 (only available at 18mm) and 3-1/3 stops faster than f5.6 (the aperture of your 18-55mm zoom at 50 or 55mm). A faster lens means a lower ISO will be needed in low-light situations. To get the same shutter speed as you'd get using ISO 400 and f1.8 will require using ISO 1600 (2 stops faster than ISO 400) and f3.5 or ISO 4000 (3-1/3 stops faster than ISO 400) and f5.6.
On your cropped sensor D3200 85mm is really too long for portraits. 50mm is just about right. On a full-frame DSLR (Nikon D600, D800, D4) a focal length of between 85mm to 105mm is considered an ideal portrait focal length. The 50mm on your D3200 has the equivalent angle of view of a 75mm lens on a full-frame DSLR so choose it for portraits.
If much of your low-light photography is going to be done indoors you might want to consider the Nikon AF-S 28mm f1.8G lens in addition to the AF-S 50mm f1.8G lens.
Personally I would get a medium tele zoom, maybe 55 to 20omm, or 70 to 300mm or similar. To my mind 85mm is too long for portraits on crop-frame, 50mm works better, and certainly 50mm would be the better high-aperture lens for a trip. Conversely 85mm is really not much extra tele beyond your 55mm, so a zoom might be of more use.