Hi! My main advice is that it is probably a bit too early for you to really weight these up – factors will change and emerge during the application process, particularly regarding the supervisors in more unquantifiable terms than how esteemed etc they are (things like how supportive they are of you/your project, how well you get on with them, how much time you think they'd have for you as a student etc). But, as someone deciding currently between 2 funded PhD offers, who didn't think about the choices before as I didn't think I'd get one let alone 2, it is probably good to start weighing things up.
However, I thought I might be able to offer some general advice – I did a HoA undergrad at Cambridge, the worked at the Courtauld as a library assistant for two years straight after. Obviously, it goes without saying that these are two of the best places to study HoA in the UK, so uni/department reputation shouldn't really come into it (and tbh shouldn't in general in terms of PhD choices).
In general vibe, you're probably looking at two very different experiences – the HoA dept at Camb is pretty tiny, but its in a bigger student community through the wider university, though Cambridge as a city is small obv. At the Courtauld, it's much bigger than the HoA dept at Cambridge, but then that's obviously it as the uni is just HoA - but you're in London, so lots of other unis etc if you want to feel like you're branching out a bit (KCL is particularly close). I knew a lot of PhD students at the Courtauld when I worked there (some worked in the library, and more through union stuff), and it seemed like a really great and supportive community. Personally, I found the HoA dept at Camb pretty claustrophobic as a student, both socially and academically (especially as an undergrad it can feel quite restrictive in terms of topics covered), but can't really compare with the student experience at the Courtauld - though I did get a bit of a more expansive vibe based on chats with some people there. In terms of funding, there's obviously a lot of ££ sloshing around at Camb, but the Courtauld seems to do really well out of CHASE, AHRC Collaborative awards, and also met a lot of phd students funded by pots outside of Research councils so there seemed to be a good set of options.
What I will say is Alyce Mahon (who I assume is the surrealism specialist at Cambridge you mention) was one of the best teachers I ever had at uni, and if you can get her to supervise you I'm sure it would be amazing. If you're talking abut Gavin Parkinson at Courtauld, I've also heard really great things about him, and knew one of his supervises p well who seemed to rate him. By virtue of having more staff at the Courtauld, they did also seem to deliver much more niche teaching than at Camb - while that doesn't really have as much bearing on a research degree, it could be worth looking beyond the 'big names' there to see if there are ones who align more closely with your topic beyond period.
Or thinking about co-supervisory teams (at both) – e.g. having one supervisor that shares a similar period to you, but another that aligns with a different element of your project (e.g. if you were looking at late surrealism in LatAm, Klara Kemp Welch would be a great co-supervisor, etc). It's also worth thinking about other potential supervisors in case your top pics are a) not interested in your topic, b) isn't accepting new phd students, etc.