I would go with the thunderstruck. It is a very good ball and it is more versital than the agent so i think it would be a better benchmark ball and it would be more in the middle of the domination and tropical where as the agent would be closer to the domination in my opinion.

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High game-300
High series-723

A benchmark ball, then why aren't you considering a Spit Fire? I own both ThunderStruck and an Agent, would trade either one of them for anything. The reason I suggest the Spit Fire is because it's going to give you a read somewhere between the two balls you considered if you were to drill them all the same. Not to mention, it will blend the lane down better then either of those pieces.

I just punched a T-Road Pearl, that's another ball you should not sleep on being a benchmark ball either. People are sleeping on this ball but I don't foresee my ThunderStruck or Fired Up butting in front of this ball right now. And my ThunderStruck solid must have 150 games on it. So you know I have enjoyed that ball.
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STAY FF MY BLCK!
Mr. Straight Ball is still praying that his ball hooks

Given what you've already bought, you're probably better served with a Thunderstruck Solid as a benchmark because the Agent may be too close to your Domination, depending on how you drill them.  I don't really use a ball as a "benchmark", because I can use just about any of them to get a good read on the lanes.  Neither is a bad choice, the T-Struck solid is just a better choice because of what you already own and are planning to own.

I use the Thunderstruck Solid, but I know different people use different types of balls.  I really really like the Thunderstruck Solid and can use it for most conditions.  I think you would have to ask someone who has used both balls and I haven't used the Agent.  

What I do is use the Thunderstruck Solid in everything but heavy oil, and I have a sanded ball for that.  I use the sanded ball for a bit and then switch to the Thunderstruck Solid ASAP.  

I think I read that most people prefer a solid as a benchmark ball.  I don't know about the Agent but I can say that the Thunderstruck Solid has a lot of hook potential but is very smooth and versatile.
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Lane 1 baby.  Lane 1 baby.  Lane 1 is on the rise and here we come baby!