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.
AlphaTech is of course completely correct. Ive come to find his advice some of the best on these forums so read it carefully birdking. but i would like to add my own experience to the mix.
i have the airport1 base station in my home. and i have one very little complaint. its only got one ethernet port (a WAN port but no LAN port). i have the system configured in the following way: DSL to router router servers pc desktops (my roommmate's) and serves apple base station apple base station serves laptops. what this means is im behind two routers (the base station is just a wireless router). This is kinda annoying. port mapping through two routers is buggy with some apps (not sure why). if i has a base station2 i could junk the router and have the base station wirelessly serve my laptops as well as serve the desktops. this is a much more simple configuration and much easier to configure and trouble shoot.
if you only want to connect wirelessly then by all means get the airport one. but if you want to use a wired connection for desktops or for your laptop in certain instances, then you will have to weigh how much a simplified system is worth. im going to buy my parents a wireless connection soon and will probably purchase an airport1 even though they could use a wired connection as well. in that instance the extra cost isnt worth the benefit, my parents wont use the net as heaviliy as i will. well actually i will probably buy an airport 2 and give my parents my airport1, solve both our problems.
so when choosing, like alpha said, you need to think about how you will use the device and if you could use the functions of the Airport2 then you need to weigh the costs/benefits.
I made the choice of not putting a router into my mix with the airport base station (1). You can pick up ethernet switches for under $100 these days (for the 8 port, 4 or 5 ports run somewhere around $40-$60 I believe). I figured out that I needed to use the uplink port in order to get the base station to talk with the DSL modem properly. Since the base station also acts as a router between my Mac systems and the peecee, I didn't need one.
I would suggest getting a network switch if you plan to connect more then one wired device to the base station (like a computer and network printer) since it is a less expensive route to take. Even with getting both parts, you should come out cheaper then the new base station, and be able to connect more LAN systems/devices to the internet.