Look at the colleges' websites and see what benefits they offer. I chose Christ Church because it has nice buildings, it offers free gym, swimming, tennis and punts, and the graduate common room was Einstein's bedroom during his time at CC (in that room we also have free coke and energy drinks, a coffee machine a 60 inches TV with PS4, etc.) Also, they shot harry potter there. Each of the colleges has pros and cons, just choose the one that you like, because academically it makes zero difference for PGs (unless you are a DPhil maybe).
Some nice perks I am aware of:
Merton has a professional coffee machine worth around 4k (idk why they decided to spend their money on that, but well :D) but the coffee is good. It is also one of the oldest colleges with nice buildings and views over CC meadows. Their chapel is also amazing.
Queens has a cute common room, with free tea, coffee, etc.
New is also where they shot Harry Potter (that scene with Malfoy on a tree) plus they have a beautiful chapel.
University has a cute little kitten plus they are very central.
St Hugh's has two cats
Harris Manchester is close to a very good affordable panini shop
Exeter has also a cute cat named Walter, you can look him up on insta. They also have amazing views over the Radcam, 10/10.
Magdalen has a huge boathouse, but they rent most of their stuff to tourists so you have to be quick.
Jesus has shit college accommodation (they are apparently building some new blocks) but it's very central and it has a cute dining hall. Also, if you like GOT, look up Jesus's coat of arms.
Lincoln is very small but it has one of the nicest libraries plus one of the current students is the heir to the Belgian throne. There is also lots of ivy, so the college looks super nice
St Catz has very cheap meals and basically that's it.
Christ Church, apart from the perks mentioned above, has a picture gallery with Van Dyck's and Da Vinci's work, a massive cathedral, and one of the best dining halls in Oxbridge in general (the meals are also fairly cheap, especially on a term ticket (like 2.40 per meal). CC also has its own cafe where they sell their own coffee (they have a huge endowment fund.)
How are people approaching choosing a specif college, any tips?