Unless you need to attach a 3rd party antenna, or you need the dialup feature, there is no reason to get the Airport Extreme Base Station over the Airport Express. But if all you want is a wireless router, there are many, much cheaper alternatives out there. Look at Netgear, Linksys, etc.
By the way, there is no thing as "Airport b." "Airport" is synonymous with 802.11b, the older, slower WiFi standard. "Airport Extreme" is 802.11g, which is faster but also backwards-compatible.