I sure the performance would be similar. I think American Turbine would be better as they have been in business longer with a long term track record. This increases the chances they would be there for you for potential warranty claims. Not that you would need any. I have an AT-309 with 300 hours on it. No problems with anything and I'm very very happy with the performance.
No personal experience with Agressor. I've heard both good and bad things about them.