You might ask an adviser from your department if they have a platform preference. I'm a proposed film major, so my department strongly encourages use of the Mac platform. However, for a biological science major, it may be different. There are labs for various platforms on campus. A list of them can be found on the following page:
UC Santa Cruz - ITS - IC - Services - Computer Labs
If you plan to work in the labs in addition to on your laptop, then you'll likely transfer files between computers on an external hard-drive. This can become an obstacle if you're working on Windows at the lab and on a Mac in your residence, or vice versa, due to the different file systems. Additionally, some software used in the sciences may only be available on one operating system or another. It is relatively painless to install Windows on an Apple machine, but installing OS X on a PC requires a bit of hackery and is a violation of Apple's licensing. This is why it's important to ask your department. They may not have any particular suggestions or requirements, in which case you can purchase whatever you fancy.
Whatever you purchase, make sure you use an education discount. Dell and Apple both have them. Also, check the prices through the bookstore as UCSC students may get even better deals on occasion. As you are leaning toward the Mac platform, it is worth noting that the MacBook Pro lineup now extends down to the 13 inch size and, with education discount via the Apple website, starts at $1099. The standard MacBook with discount starts at $950. Additionally, Apple is currently offering free iPod Touches with the purchase of its computers. Personally, I own a MacBook Pro, but I purchased it prior to the development of the unibody design. I like that it's made of metal and it has been very sturdy, but I haven't heard that the plastic MacBooks are prone to damage. Most laptops are made of plastic and it's a fine material to use. Still, the new Pro's are quite solid. On the other side of the coin, Dell has definitely improved the overall design of its laptops and Windows 7 looks to be a solid improvement over Vista and good competition for Apple's OS X.
Whatever you decide to purchase, you'll probably be okay.