Several people on matrixowners.com and matrixvibe.net have the Injen intake and love it.
If you look at the dyno's for both, the AEM offers better peek HP, however it suffers a horrible torque loss at low RPM. The Injen is just a shade short of the AEM for peek HP, however it has no torque loss. I would get the Injen.
After I get my TRD springs, the Injen is next on the list for me. I have a friend who took me to one of his Eclipse meets (mitsu guy) and his small group of 5-6 cars ALL had the Injen intake, everyone swore by that brand.
The dyno proves the results, unlike some companies (stillen) who claim HP but don't back it up.
Just my $.02 worth.