comparing the 07 v8 to prior year STs for ride, handling, power, acceleration, etc. cannot be done. the 07 is a big jump compared to former years (another generation, really) in those attributes. I had an 03 ST and now have the 07 v8 (both were 4x4) and even my parents were impressed when I let them drive it for a week when they visited. My dad could not get over the agility and turning radius of the 07, in the 03 the turning radius SUCKED.
ride em' both and make a decision. people who really like the frontier will tell you to buy it. people who really like the ST will tell you to buy it. objectively, if that is possible, the ST is a better choice. The frontier is another truck. The ST is its own phenomenon.