I went into my catalog and compared some nightscapes from the Rokinon and the 16-35 f/4. The Tamron has less distortion towards the edges, and that caused more stretching on the Rokinon. The performance from that Tamron is good enough that I feel I could let the Rokinon go in favor of the Tamron and not lose anything.
The 16-35 f/4L was simply not as good for astraphotography, in my opinion. I never got the starpoints to render as sharply, and, of course, you have only half the available light to work with.
One big plus is that the focus window is accurate on the Tamron. I just put it on infinity before going out to shoot and got a very sharp result.