The custom tunes we build for the car will be the same for either device. BDX adds the feature of receiving custom tunes over Wifi without the need to hook up to a PC to load tunes we send over email. Downside is that is something extra to click through when loading tunes to the vehicle whether or not you want to sync to the cloud. X4 can be easier to use, but you'll need access to a Windows PC to load tunes received via email. X4 is more popular.