I have both and I can say without getting too specific with anything before I say it I will have to say the T-Road Pearl is a better ball overall because of its versatility. The Spitfire is more of a condition specific ball, for me at least.
I have a low axis which helps both of these balls get through the fronts pretty well, but the T-Road Pearl will get through burnt heads with ease. The spitfire will rev up quicker but will give you a more even, arcy reaction. The T-Road will save a ton for the backend and will give you even more length with your axis and based on your layout. Since you are bowling on carrydown I wouldn't get too weak of a drilling because it might shoot a little too long for you. You would probably still be able to get away with a drilling under the fingers.
Like I said the T-Road will give you great versatility. When your Agent is too much you will be able to stay inside with this ball, maybe get a tad softer and it will rev up and destroy the pins. Or, if you want to go straighter from out this ball will let you do that all night long. If you're on a lot of carrydown you might want to put a touch more surface on it, however.
The Spitfire is going to be more of an earlier rolling, arcy reaction and since you're looking for retaining energy for a big backend I wouldn't recommend it for that specific reason. It's still an amazing ball but just maybe not what you're looking for. If you go with the spitfire you might have to move a little more inside of where you were with the Agent so you can catch a bit more head oil to keep it online in the midlane. From my experience, the Spitfire isn't a huge fan of carrydown, but it depends how much you're on.
If you want something that will handle a good bit of carrydown but still have an amazing backend, I would personally recommend the T-Road Solid. That ball is money, that's all I have to say, you can't beat it.
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