I use the nkc freeduino and am very happy with it and all the other boards I have purchased from them. You can buy directly from their web site if you want although I think the prices are similar to their ebay shop.
I have not seen the Freeduino SB in the flesh but the mini usb jack and layout for optional aref trimpot are very nice if you want these features. If you are going to be using the board with external power note that its rated for 6v only.
You can find the answer to your Xbee + usb question here:
have fun!