I have both the Ecometer and the scan gauge. After using both extensively, I decided to mount the Ecometer permenantly.
I chose the Ecometer over the Scangauge purely for ease of use; The Scangauge has way too much information to be used just for fuel economy. If you are so detail oriented that you need every bit of data that is available on OBDII the the Scangauge is for you.
If you just want the indicators on a gauge that will help you improve your driving habits, then the Ecometer is for you.
I bought my Scangauge for $159 including shipping, and the Ecometer for $69 including shipping.
The Ecometer is less obtrusive than the Scangauge when trying to find a mounting location. The easy to read and well-lit face on the Ecometer makes quick work of interpreting how your driving action translate into fuel economy (or detriment).
The scangauge will tell you more about your engine's performance than you were even wondering about. I will keep the Scangauge and use it when I have an issue with my car's performance, but I'll use the Ecometer for improving my MPG. 49.7 last tank, must be doing something right.