I have both the Scangauge and the Ultragauge. Scangauge (SG) is on my 4runner, Ultra Gauge (UG) is on my DD Mazda Protege sedan.
IMO the Ultragauge is more versatile and the interface is more modern looking. On the downside, the UG doesn't show transmission temperatures, which the SG does. The SG also has better mounting options because it has a network cable plug (actually two of them) whereas on the UG the cable is permanently attached to the unit. Also on the UG you have to be able to access the back of the unit in order to operate it, while on the SG all the controls are on the front, which means you can flush-mount it.
The "fill up drill" with the SG is a little easier, but the UG has a better calibration system.
Either one work fine for showing things like engine temp, distance to empty, average and instant MPG, etc.
UG is ~half the cost of the SG but the SG is probably a little higher quality (hard to say but that's how it feels.)