Depends on how much top-end you need.
How fast do you ride, and what cadence range do you prefer to ride at?
How strong are the groups you ride in: when they're going tempo on the flats in calm conditions, what sorts of speeds are you looking at?


The 11-34 cassette doesn't cut off any top-end compared with an 11-28 cassette, because highest cog on both cassettes is 11-tooth.

52T is about 8% bigger than 48T. So, at any given speed, when you're in your highest gear, you'll be spinning your legs about 8% faster with a 48T.