Well, the thing about University College is that its main attraction (the castle, obviously) holds roughly 100 students. As far as the rest of its accommodation goes, one may as well reside in any of the other colleges. Hatfield, while lacking any singularly remarkable accommodation as the castle, appears (from my distant position) to have a greater quantity of accommodation dating back to the 1800's or beyond. In other words, with University it looks like you're either in the incredible 1,000 year old castle or a residential block from the Cold War era, whereas most Hatfield accommodation is from a more elegant age (though it appears to have its share of nasty Cold War architecture as well).
Actually, upon revisiting the website to refresh my memory, I felt more comfortable with Hatfield's facilities largely because there's simply more information about them. Hatfield gives out more information in general (right down to a sample catering menu) whereas University is relatively vague. I like to know exactly what I'm getting into, and I feel that I know much about Hatfield and like what I see.
Besides, I kind of like the idea of being "on the dark side" relative to University College. (I'm referring, of course, to the University-Hatfield rivalry and everyone's tendency to side with University.) I'm a rebel like dat