Thwaite is lovely grounds, but is much more expensive, so more of your student loan on rent and less on other very important stuff... like shoes... and social! Its a different type of community to the lawns.
I lived at the Lawns and really loved my time there. Yeah ok the food isnt gourmet, and ok the buildings may not look HUGELY appealing, but the atmosphere is great and its where the large majority of the first years are. when you're meeting people on your course - chances are the most of them will be at the lawns.