Check out
this thread for reviews of the MaxiTrip...
It looks like the MaxiTrip can't compete with the overall quality of the scangauge. It's a cheap alternative if you really can't spend the money now, but you'll probably end up regretting not spending the extra money in a few months.
First though, what kind of car do you have? The SGII only works on OBDII cars ('96+), so if you have an older one, then you'll have to look for something else anyway.