Mick, if you can only choose one out of those two, I'd go with the 70-200mm simply because it's more versatile and would allow you to frame things just right for astro.
The 135mm is excellent indeed and I love using it for astro, but if I had to choose, I'd pick up the 70-200mm first.
It's a top performer and the quality is very close to a good prime.
I own both and I usually consider the 70-200mm first unless what I am imaging will fit perfectly in the 135mm.
The 135mm is superb for terrestrial shots though, it's one of my favourite lenses.
But like I said if I could only pick one, I'd pick the 70-200mm.
