Thanks for the replies, all. (I tried to respond yesterday, but the reply buttons were returning a javascript error for some reason.)
So, I see 3 reasons to choose the new Express over the 4th Gen. Extreme: Smaller form factor; runs cooler; comes with phone support and warranty. Having said all that, I ended up being able to get the A1354 Extreme for $70, so I took it.
Which leads me to two more questions for you fine folks, if'n you don't mind:
1. If I set up a wireless network on my ISP's modem/router and connect my Extreme to it wirelessly, will I still be able to enjoy the Extreme's simultaneous dual-band functionality and its dual-network functionality?
2. Can my girlfriend and I both do wireless Time Machine backups to an external drive connected to the Extreme -- and if so, are there any configuration changes required, or can I just connect the drive and the partitions with our TM backups will appear automagically in Finder?
Thanks in advance, and forgive me if either of these questions are on the stupid/uninformed side. I've done some digging on the interwebs, but don't yet have physical posession of the base station, so...