The best thing about the SCT X4/SF4
is its ability to datalog and view parameters on the hand held's screen. Then you can connect it to you laptop via WiFi to send datalogs to your tuner. Plus the X4 is a lot smaller than the X3, which is just plain convenient.
If you already have the SCT X3, than I'd stick with it. Unless you enjoin viewing the engines parameters frequently, there's no need for the switch. As they both can hold the exact same tunes.
Hope this helps! Let me know if you have more questions