UPDATE - January 31, 2018:

I've gone through and uploaded some new test images to compare the performance of both lenses for Milky Way photography.  

First Look - RAW Photos

This comparison shows both lenses straight out of camera, no editing has been done.  The camera settings were f/2.8, ISO 1600, 20 seconds.  The Nikon was at 14mm and the Tamron at 15mm.  The main difference I notice is that the Tamron is slightly darker across the entire frame.

Tamron 15-30

Nikon 14-24

After Lens Profile Corrections

Next, I enabled the Lens Profile Corrections in Adobe Camera RAW.  This automatically corrected the vignette and distortion on both images.  

Tamron 15-30

Nikon 14-24

Color Balance

For this comparison, I increased the Exposure by about 3 Stops in ACR and increased the Saturation by 80 points.  This gives a clear look at the difference in color balance between the two lenses.  Both images were shot at 4600K with a +8 Purple Tint.  The Nikon clearly has a cooler color balance, and in my opinion it looks better.

Tamron 15-30

Nikon 14-24

Coma

Both images were cropped heavily into the upper left corner of the image.  Unfortunately my shutter speed was a bit long, at 20 seconds, so there is some motion blur.  However, it's clear that both lenses perform extraordinarily well in this test.  There's some minor chromatic abberation on both, but nothing to be concerned about.  Astigmatism is not a problem here either.  If you want to see some bad coma, check out the Sigma 35mm Art wide open!

Tamron 15-30

Nikon 14-24

Round 2

RAW

Later on in the night, I took some more comparison images.  This initial test shows just the RAW photos again, no edits were done.  I did increase the ISO from 1600 to 6400.

Tamron 15-30

Nikon 14-24

After Editing

In this final comparison I applied the lens correction profiles, increased the exposure, increased the shadows, and increased the Saturation in ACR.  Both images had the exact same edits applied.

Tamron 15-30

Nikon 14-24

Final Thoughts

Before I begin, you can look at the Comparison Gallery and download the full-res JPEGs for each comparison.  Now, let's break down the final verdict into a few important points.

Light Transmission

As we saw in the comparison images, the Nikon produces brighter images than the Tamron.  The DXO mark tests confirm that the Nikon does gather more light than the Tamron.  According to DXO, the Nikon is rated at 3 TStops while the Tamron is rated at 3.3 TStops.  If you are unfamiliar with TStops, I'll explain.  Basically, TStops measure the actual amount of light that passes through the lens.  Normally, the fStop and TStop should align.  I.E. an f/2.8 lens transmits T/2.8 of light.  However, most lenses don't work that well.  In this case, the Tamron is actually a 1/3 Stop darker than the Nikon and a 1/2 Stop darker than advertised (at f/2.8.)  Watch Tony Northrup's TStop video for more information.

Coma

Both lenses perform extraordinarily well!  There is a slight amount of Chromatic Aberration with each lens, but nothing to be concerned about.  It can be easily fixed in Camera RAW or Lightroom.

Vignette

Again, this seems like a tie.  Both the Nikon and Tamron have a small amount of vignette in the corners.  After applying the Lens Profile Correction, it is no longer an issue.

Price

The Nikon is currently selling for $1,900, and the Tamron is currently selling for $1,200.  Clearly the Tamron is the winner here, with a savings of $700!

Other Considerations

During the day, the Tamron actually out-performs the Nikon.  I've seen that the front element has better moisture resistance and handles lens flare a lot better.  I can tell you from personal experience that lens flare is a big problem with the Nikon.  It seems no matter where or when I'm shooting, I'm always dealing with some annoying flaring on the Nikon.  The Tamron also has built in Vibration Control, a nice feature that the Nikon is lacking.  The Nikon does have that extra 1mm on the wide end though, which can make a difference.  If you plan to do real estate photography too, this might be something to keep in mind, as that 1mm will come in handy when photographing tight rooms.

Final Verdict

All things considered, the Tamron is the winner here!  The Tamron 15-30mm has Vibration Control, better moisture resistance, much better flare resistance, costs $700 less, and performs at the same level as the Nikon 14-24mm at night.  The only downsides are the Tamron's worse TStop rating and loss of 1mm at the wide-end.  In the grand scheme of things, these are both very minor points.  

Regardless which lens you decide to get, it ultimately comes down to the photographer behind the camera.  If you want to learn more about Milky Way photography, check out my Milky Way Tutorial.  This covers everything you need to know!  You can also check out my YouTube page for some video tutorials on editing the Milky Way, including: Removing Hot Pixels, Fixing Amp Glow, Photo Stacking to Reduce Noise, and a general Milky Way editing video.  I would highly recommend all photographers who are interested in Milk Way photography look into Star Trackers too.  These devices make a massive impact on your image quality!  Check out my Star Tracker Tutorial and Buying Guide for more information.

Also, if you plan to use either lens during the day, you should consider getting some 150mm filters.  This will allow you to make use of these great lenses for long exposure or waterfall photography.  I have reviewed the Haida 150mm Filter Holder along with their 6 Stop ND and Polarizer.  In short, the filters perform incredibly well!  I also reviewed the ProGrey G-150Z Filter Holder and 10 Stop ND.  I am a huge fan of the G-150Z and highly recommend this filter holder, regardless which filters or lens you buy.

Finally, here's a Star Trails photo I captured with the Tamron 15-30mm.  The full moon illuminated the landscape nicely!

Observatory Park