What places can best support your specific research interests and what personal factors are important to your decision?
I'm in exoplanets so I mostly know UCSC for its exoplanets program, but I know they are great at astronomy in general too. The current 1st year graduate class is *17* students. There are only 35-40 grad students total so the current 1st year class (2nd year when you get there) is more than half the department. This is due to more than expected number of students accepting offers and some newly recruited professors bringing their old students with them. My experience with the department there is that everyone is very collegial and I feel like it would be an awesome place to work. I actually applied to take postdoc fellowships there. That didn't work out and I did end up with a postdoc offer at another great place, but UCSC astro is certainly a place I could be very happy at. In addition, the UCSC Astro dept is one of the most productive and strongest departments at UCSC. It certainly wields a lot of clout at the school. If you are an observer, access to UCO Lick and the Keck telescopes (through the UC system) is great.
I have also visited Arizona a few years ago but did not spend much time in their Astro department (I visited the Lunar and Planetary Lab / Planetary Science department). I don't know much about the culture other than the letter posted above, which does seem scary but based on my experience with grad schools, I can see how it could be taken out of context and while the letter still raises legitimate concerns, I also wouldn't let the letter without context affect my decision. If you haven't talked to students there yet about quality of life, then it would be a good idea to do so! Steward Observatory is also a very strong astro department! If you are an observer, access to the MagAO and other southern hemisphere telescopes is great.
So, maybe that's not super helpful to hear great things about both schools, but the good news is that in terms of astro research quality, I don't think you can really go wrong. I would make the decision based on your specific research interests and your personal interests. I feel that Santa Cruz, CA and Tucson, AZ are very very different places to live so it just depends on what you like more. I hope you did have a chance to visit both cities.
For me, I would love living in Santa Cruz. It is very expensive though, but the UCSC campus being in a forest is so lovely to me. I found Tucson way too hot and dry for my personal tastes. However, grad students can afford to buy houses there or rent super nice luxury apartments/condos. So there are certainly trade-offs.