(and sorry for my english)
Every time I see this, its almost guaranteed that the poster writes better than 90% of my US customers.
I use these RN-XV wifi modules:https://www.sparkfun.com/products/10822
$35, neat set of features, fits into an Xbee socket. I guess it would depend on the application. Xbee's are nice if you need to make your own network, WiFi is nice if the network is already there. There's also a lot of cheap consumer WiFi products that you can connect to with a WiFi module.
EDIT: Re-reading your post, this seems almost exactly the same setup that I have. I have a Raspberry Pi running Apache, PHP, and MySQL, and a bunch of sensors connected to RN-XV modules that periodically send data to the RPi Server.