The most useful gauge to me is trip average. My old SG-II (that's mine on the bottom in that picture) doesn't have AVG as one of the gauges. I usually run it on the Current Trip screen with only the trip average showing. It seems to work.
Having the instant mpg on there was a definite bonus. The graph... it's pretty, but didn't add much.
The menu requires many more clicks to get anywhere. With only two items displaying, it has to be that way.
It refreshes faster. The newer -II's might as well, but I don't know. That should be more accurate with varied driving like P&G.
Biggest bonus is the lower price.