That's a hard question to answer because it is gear dependent and there are valid points on both sides of the issue. Assuming that all amps are made by the same manufacturer, it's pretty common to find that the 500 X 2 amp would have better quality parts resulting in better sound. On the other hand, running vertically bi-amped means that you are not subjecting your mid and upper frequencies to back EMF from the woofers.
There is a tradeoff however because, as I remember, I've heard owners comment on the Aerials responding well to bi-amping. My speakers are tough to control on the bottom end and I found that vertical bi-amping was the best way to accomplish it. If I were you since you already have the Citation, if you haven't already, why don't you hook it up and see how it sounds? If you don't like it, or feel you have enough power, then you'll probably want to upgrade anyways, even if it's going to 2 500 X 2 amps.
There is a tradeoff however because, as I remember, I've heard owners comment on the Aerials responding well to bi-amping. My speakers are tough to control on the bottom end and I found that vertical bi-amping was the best way to accomplish it. If I were you since you already have the Citation, if you haven't already, why don't you hook it up and see how it sounds? If you don't like it, or feel you have enough power, then you'll probably want to upgrade anyways, even if it's going to 2 500 X 2 amps.