Gizzo wrote:

....

1. format it in fat32. access from PC and Mac thru USB/Firewire. Pro:
easy.

Another pro is that you needn't leave it connected much of the time,
thus both reducing potential exposure to corruption by errant or
malicious software and allowing you to keep it physically separate from
the primary material that you're backing up.

Con: performance issues?

Probably not serious ones (though you might want to defragment it once
in a while if performance deteriorates over time - depends on how you're
using it): while USB 2 can't stream data to/from a contemporary drive
as fast as the drive can handle it (typically 40 - 80 MB/sec), it should
be able to handle around 30 MB/sec.

2. format it in HFS. install macdrive in PC. access thru USB/Fireware.
Pro: whatever benefits of formating with HFS (I'm not very familiar
since I'm a mac newbie and initial research indicates there are
inherent benefits as there are with NTFS over Fat32.

Exactly which of these do you think would be significant to your planned
use of this drive?

Con: I have no
experience with HFS nor Macdrive to judge their reliability. and $50
on Macdrive.
3. no need to reformat or repartition. Share the drive using native
network tools in both PC and Mac. Pros: no reformating. Con: I don't
know how to do it in a home environment of 1 pc, 1 mac and 1 wireless
router.

If you're using wireless don't plan on getting too much bandwidth
through it from the machine to which the backup drive isn't connected
directly. For that matter, even wired Ethernet would be the main
bottleneck unless it was Gigabit.

- bill