The ScanGauge II is definitely the better way to go, unless you really want another gauge as a backup, the SG will cover your needs.
Shoot, you get multiple odometers, time and miles until you're out of gas, all sorts of gas mileage stats, or if that doesn't interest you, you get over 30 stats you can read out, some of which you'd have a hard time putting a gauge on. (Throttle position, air intake temperature, etc)
Also, the ScanGauge II beats the XCal for code reading hands-down, since the ScanGauge will always be in your ride and connected, so you can check and clear (or store) codes while driving.
When my CEL magically came on 2 years ago, I was able to check at the stop light for the code, and recognize them as the evil elbow codes...and so I was able to do a ghetto repair, and then a few months later suck it up and buy the Ford replacement part
Also, you can set the Gauge to all kinds a purty colors to boot. :banana: