Like many others I need an external HDD for Time Machine. This will be a first time external HDD so advice needed. In today's Best Buy ad there are several external hard drives advertised. There are two Western Digital drives that are both $129. The first is the 500GB 3.5" My Book Essential 2.0 External Hard Drive. The second is the 320GB 3.5" My Book Home Edition USB 2.0 / Firewire 400 and eSATA Interface External Hard Drive. Which should I go for? I will be using this for two Macs. The first is a G5 iMac with approx 100GB used (160GB total drive). The second is a MacBook C2D with approx 50GB used (80GB total drive). What is more important, HDD space or having Firewire? Thanks in advance!