After a quick look into these bikes, Ill say the Aggressor has a very slightly higher end component spec. The Shifters however are one step lower than the shifters on the Frontier but everything else is a step above.

My suggestion would be the Aggressor. Id also say check into the Avalanche 3.0 Disc as these can be had for cheap at Performance Bike. It has a better frame and components all around. I have built and ridden both of these bikes and will say the Avalanche is a better quality bike all around. I work in a Performance Bike and as much as I dislike that shop, they do good things. Find a local one, ride the Aggressor and the Avalanche and choose from there. If you are not satisfied with one after purchase, they will take it back and exchange it for the other bike you were interested in.