At Touchtone, we prefer mirroring.
With a parity set, if 2 drives in the same parity set (in
your case, the ONLY parity set) go down, you're "up a lazy
river without a you-know-what." But with full mirroring,
you'd have to lose both drives of a mirrored pair for that
to happen, and even with only 2 mirrored pairs, the odds
are against a 2-drive failure taking out both drives of
the same pair.
If you have lots and lots of free space, to where you
could afford to give up 4G, you could even go with just
buying one more 8G for now.