Different people will tell you different things. My advice to you is do some research on each type and decide what you want in a pistol. I own an ACP but if I were to buy another pistol it would be the Overlord because of the looks, delrin bolt, and RRM (rapid release magazine).
Just depends on what you are looking to use it for. The G2 has more after market support but the ACP is far lighter. Both are quality markers and the design of each is comparible.
Personally, I own a G1 Zeus and the ACP. For sidearm work, I like the ACP because it doesn't drag my holster all over the place. I actually got to the point where I didn't even notice it...something that never happened with the heavier G1. However, there is no rapid reloading system out YET for the ACP and the Zeus with a quick cap has the advantage there. Both can take aftermarket barrels (G2 comes with one) and both can take a bottom line.