My humble opinion is that's not a great price. I believe that's about £10 off the usual price straight from apple which is better than nothing but not great. If you want a bargain then you probably need to either be a student or buy a refurb model.
I'm not sure whether you're asking for our opinion on whether to buy an iPad 3 instead - but I'm going to give it to you anyway! I think you should at least wait until the iPad 3 is unveiled if it's an option you're considering... it's difficult to make a decision without all the facts.
Having said that they're completely different machines, one is a tablet - the very essence of portable computing, the other a desktop - designed to never leave your desk. What do you need it for? And have you factored in the cost of the peripherals for the mini? (i.e. monitor, optical drive, mouse, keyboard etc.)