If you are anywhere near a Costco store, they keep Bosch at reasonable prices. I believe that Bosch and Varta are now part of the same group. The batteries look identical apart from the label. I got a Varta from a local factor recently for only about £5 more than Costco wanted for the same with a Bosch label so it wasn't worth the trip to Costco.
I would get the biggest battery that can fit in the space provided. As a battery ages, its capacity reduces. So if you get a big one, after two or three years it will still have the capacity of the standard battery. You will notice a large difference in price from the common sizes to the uncommon ones, so get the biggest that will fit for a reasonable price.
You can find all the sizes etc here
http://rb-aa.bosch.com/aa-batterien/gb/en/start/index.htmIf you have the money (and the room for the S6002), the S6 range with AGM are supposed to be good.
You should be able to get a Bosch S5007 or Varta equivalent or larger version (I think the S5010 is the next common size) for £80 - 100. I got a Varta Silver H3 (same as Bosch S5013) for a neighbour for less than £100 recently.