Well, for $200 you get an extra 66mhz and 20GB of hard drive space. The speed difference isn't too big, and you can get external hard drive space for much cheaper.
I'd go with the 933, and spend $150-$200 on a nice sized FireWire hard drive if you need the extra space. This way, you have a quick, easy backup solution and you can just plug it in to another computer to share files. You will also be able to "take it with you" when you buy your next computer.