The C9.25 has a flatter field of view (due to the size ratio of its primary mirror to its corrector plate) - making it better for astro-photography, the C11 has more light grasp - so its a trade off. Do you want more light for fainter objects or a flatter field of view? Better explained here:
http://www.cloudynights.com/item.php?item_id=1528
But a side by side comparison of the two is worth reading too, here:
http://www.cloudynights.com/item.php?item_id=1464