Catalyst is more expensive by like 400. I prefer to get boots with my tear if I come back home with enough money for catalyst. It is indeed a great safe choice, but I paid the IP for flat mana so that I could be as abusive of a solo lane as possible, and even without the extra health, Ryze can trade hits pretty well.

Also, my philosophy with Ryze is to be a Tanky DPS that tunnels squishies. A fast Abyssal can make or break that because Spell Flux makes stacking flat reductions that much more impressive. My core usually extends like this: Abyssal>Hextech Revolver>Archangels>Will of the Ancients