I was in the exact same position as you last year haha. I went for RH over Brunel because I preferred the campus, thought the uni was "better" and Royal Holloway gave me a lower offer (ABB as opposed to AAB), although I'm pretty certain that the Brunel course ended up in Clearing.
Pros:
-The course is pretty interesting and varied for the most part (you pick modules in the 2nd year, everyone doing Management is together in the 1st year).
-The campus is great
-Management lectures are huge (400-ish people) so they're all in the Windsor building, which is great.
Cons:
-Lots of International Students on the course who don't seem to want to mix with other people that much (not all of course).
-Egham is pretty quiet if you're used to a big city. My GF goes to Brunel and prefers it there because it's far more urban, although it's only 40 mins to Waterloo from Egham, and Egham High Street has all the essentials.
But yeah, you won't be making a terrible decision no matter which uni you choose. I personally prefer the Victorian leafiness of Royal Holloway, but Brunel is undeniably better for going out/shopping. That said, about once a month I'll make an effort to go to one of the big clubs in London, and Staines is only a few minutes away on the train or bus.
Hope I helped, good luck with your A Levels