I don't think Lancaster will equal leeds at all when it comes to social or music scene, actually. Far from it. Lancaster doesn't have much 'on offer', however what it does have is lots of room for you to make your own scene. It's not too much of a bubble, and it really is what you make of it - if you want a really good night you can probably have it, but if you want a really good city night, you might end up wanting to go to Manchester after a few tries in Lancs. It can be great, but it won't be close to Leeds on that level. But you'll be in the same boat as so many other students wanting a good time, and you will not be at a total loss for that, don't worry. Lancaster is getting a large Primark next year, you may be pleased to read.
As MM said otherwise, though, there's nothing wrong with turning down the unconditional just because it may seem a little bit of a waste, but only if you're sure that you want to go for Leeds. Have you visited Lancaster? It will charm you if you do (if you come in the sun...)! (: