For most manufacturers, you will find similar bikes for similar prices. Test ride them, and see if you prefer one over the other. Also, while test riding, try to determine how helpful the staff is at each shop, and lean heavily toward buying at the most helpful shop.

The two you link to are different in at least one way, in that the Cannondale has disc brakes (Trek also has FX models with disc brakes). For most of us, disc brakes are of minimal value unless our riding conditions mandate otherwise... For example if you plan to ride in the rain, then disc brakes are better at stopping during wet conditions. Me, I ride when the weather is nice, and if I get caught in a down pour, I just slow down earlier to accommodate the wet brakes.