It all depends on what you will be connecting to. The new one fully supports AOHell, has a WAN and LAN ethernet port, and other internal changes (firewall for one) and supports up to 50 users. If you don't care about all that, then go for the first one.
I have the first base station at home in the following configuration.
DSL modem plugged into the uplink port on my Netgear 8 port network switch. Airport base station plugged in next to the uplink port. The base station is set up to go online for me 24/7 and give out IP addresses via DHCP. I then have a G4 tower as well as my own build pc plugged into the switch. My laptop has the airport card installed in it. All three systems can go online at the same time, and have. I have tweaked the settings some to allow for a higher bandwidth between my laptop and the base station. If you already have some networking hardware, or plan to have less then 10 people on the base station at a time, then go with the cheaper one. If you think you will have between 10 and 50 people, get the new one.
I don't think that the firewall is too much of an issue for Mac people, since most of the stuff that comes over in a nasty way targets peecee's.
I talked with one of the people at the Apple store when I was getting my iPod, and for normal home usage, the first base station is more then sufficient. Corporate, or larger networks would benefit from the newer one.