Both units take dual laptop size hard drives, and both are bus-powered (Firewire or Firewire+eSATA), which means you don’t have to use a power cord. That’s way-cool, especially for travel with a laptop!
Both units as “quad interface”, so they can be connected with Firewire 800/400, USB 2, or eSATA (fastest).
Both units can run as a RAID-1 mirror, so that if one drive fails, things just keep on working. If a drive should fail, you’ll want to replace the failed drive, but the point is that you still have all your stuff fully accessible and working. Backup your stuff (just in case), replace the failed drive, and keep going. OWC will of course help should a failure occur— they have 24X7 support.
Either unit can also be configured to run as a RAID-0 stripe, for higher performance (especially with eSATA), and twice the capacity of a single drive, but always remember that failure of either drive in a RAID-0 stripe kills the volume. Backups should be more frequent when using a striped volume, and remember that Time Machine does not backup external drives. For travel, a mirror configuration is the smart move, because the risk of impacts affecting the hard drives is a risk even beyond ordinary drive failure.
The ultimate choice is dual solid state drives, which are more impact resistant, and have no moving parts to fail. Anyone with critical data and/or high performance needs should consider dual SSDs preferentially to hard drives.
For hard drives, go with the 7200rpm 750GB + 750GB configuration. That yields 750GB of usable capacity in a RAID-1 mirror configuration.
A 5400rpm 1TB + 1TB model is also available (1TB usable capacity as a RAID-1 mirror), but that’s a 5400 rpm model, so performance is lower. Stick to the 750+750 model unless you really need the extra space.