A Pod XT will let you record over usb, so if all you need to record is your guitar, or hell, even a mic adapted to plug into the 1/4" jack on the Pod XT to sing through (with GearBox plugins you can get some killer preamp modeling!), then it can suit you just fine.
I don't really think there's any special advantage to the MBox, it's overpriced, and unless you NEED to run/learn Pro Tools, there's a lot better values out there for the money. What recording software are you using? Mac or PC?