Well I have thrown both balls. I own the Shift and my friend has the agent(similar drilling's). I've found that(I assume your wanting this for an THS) the agent is more for a down and in shot unless maybe you have the revs to bring it back, and the shift is more of an inside angle. For example, I can throw the shift over 12-13 out to 5 and will come roaring back. As for the Agent, I have to throw it over 9-10 with no swing for a dead flush and if I get it out a little more a light mixer will do fine. The Shift is definitely the stronger of the balls. I've had much success with it so far, highest series 782 and a couple 288's(stupid mistakes). A lot of guys I talk to are very unhappy about their NV after about 30 games. Storm is really putting out a better product. Hope this helps.
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Arsenal:

Storm Shift
Blazing Inferno
Morich Mayhem
Vapor Zone
Maxim Captain Midnight