I'd also go for the GBX, since it's the best supported Backup Unit on the net (got one myself ;) )... but just in case you want to inform yourself about the other options www.subport.org is a good starting point (the reviews are in the 'hardware' section). Netmars' e-merger for example looks like an interesting alternative (at least if you like Pokémon ;) ), because it features an integrated real-time clock.
--Matthias