I play SR (500) and not only are they GREAT bang-for-buck, but they're also awesome in every other way. If you can get one with the bartolinis go with them - they're soooo transparent.
The Sterlings are also great basses although I've only played in music shops.
Really though, it depends on what you want, and how long you're going to keep it.
If you want a beautiful piece of wood and a clear clear sound, go with the SR - but the resale price will be shot if you're going to re-upgrade next year.
If you want the MM sound, the Sterlings are great basses for any level of player, decent resale price if you're going to upgrade.
If you want a Fender sound (either), then £600 isn't going to go massively far, but you'll be able to resell for practically the same price if you decide to change your mind.