So why 250gb? Why not 500 or 750? At ~$200, a 750gb drive isn't seriously breaking the bank. Well the simple matter of fact is that hard drives fail. Whether it's a manufacturer defect, bad maintenance (dust, hot case), or just plain old age (although some can last a looong time, they should really be replaced every 2-3 years), at some point your hard drive will fail.
So what do you want to do? Replace 250gb or 750gb? And don't even get me started on the new terabyte drives. It's also easier to back up a smaller hard drive. Heck, I might even just get another 150gb or few - I have two internal drive bays left and an external SATA port. Losing 150gb vs. a terabyte of data thanks to a drive failure sounds way better to me.