I know (or think I know) that both devices can do this.
Basically I have the ISP provided router, which has wifi, but it doesn't work well. That gives wifi to most of the house, and then on the other side, I have an airport extreme that then hooks up to an xbox, ps3, etc, and provides wireless to the other side of house.
I want to replace the ISP wifi (well, turn off the wifi on the ISP router) with an Apple Base Station, to better extend the network. I would purchase either an extreme or express, and plug it into the router, and disable wifi on the router.
Is there a benefit to using one or the other? Can an Express have a Hard Drive attached to it for TimeMachine? Can an Extreme extend a dual band network from another extreme?
Update: I already have one extreme, and very use to the software. Just wondering if it would be better to get another extreme to extend the network, or just an express. The extreme runs dual band, has a HDD connected. Its mostly in a bridge mode, the ISP router still handles DHCP, etc. Just want to disable wifi on the router, and add a second base station.