Actually just did a ton of research on this myself and found this great article: http://www.lonelyspeck.com/lenses-for-milky-way-photography/
It give's you the run down of the different lens options and what actually goes into taking pictures of the Milky Way. Just got the lens I ordered for this the other day. Got myself a Sigma 30mm f/1.4 which, was very cost effective at $500. Took a few test shots last night and it blew my mind the amount of detail in the stars with just a 15 second shutter.
To build on my answer a bit, using a 50mm lens the maximum exposure time that you won't get star trails for is around 10 seconds for a full-frame camera, and for an APS-C camera it is around 7 seconds. The only way to avoid that is by tracking, so not sure how you can not get star trails with a longer exposure. With my 30mm and my canon APS-C camera the max time is around 15 seconds, and if I zoom in all the way I can see a slight star trail. In the end... the shorter the focal length the longer the exposure time that you can have without getting star trails. Obviously also the lower the f-stop the better.
If you read up on the astro score, your 50mm f/1.4 lens gets a better score then if you shot at 17mm f/2.8. In your case since you have both lenses, go outside and try them out and see which pictures come out better. That is really the best way to tell. Also, whenever I am taking astro pictures the first thing I do is set my ISO to 100 because I would rather have less noise in the picture with slight star trails than a really noisy picture.
As a side note, I was messing around with my pictures post-production with my new Sigma lens and bumped the clarity to 100 and it honestly didn't add that much more noise to my shots. So all-in-all if you are willing to spend $500 I would strongly suggest the Sigma. Was probably the best astro lens I could find for the price.
Here is one of the shots I took at f/1.4 ISO 100
Hope this helps!