They usually are. I know I had to pay a few quid more to get a Firewire one. Obviously if you have a spare slot then firewire's convenient. But USB2 is usually slightly faster, especially for smaller files. Neither of them is significantly slower than the internal hard drive. In fact, since the MacMinis use laptop hard drives, a 'normal' external hard drive will probably be faster even. As Jon said, try and get one which doesn't need a power supply if you plan on taking it places. When you get it, you will need to make sure it's formatted in FAT32 so that both PCs and Macs can read and write to it.