if you go with the G, you will drop your network to G speeds if it's connected. which for just internet should be fine, the only time you should notice a difference between G and N is when doing file transfers between computers on your network.
Any G device that connects to a N network (in G compatibility mode) will drop the speed to G, so if you have an iPhone or something you've already dropped to G speeds.
But G is fine for doing airTunes. I use my old G express for just that.
Also if you want to have a device (game console or something) join your wireless network, you can have a N express join your network, and share the connection on the ethernet port, the G will not do this (you can do it on a G in WDS mode, but that cuts down your wireless speeds)