Cast iron generally wins, as it's usually more massive and thus retains
heat far better than the smaller-diameter porcelain grills.

Cast, however, tends to get gunked up more quickly and doesn't really
want to be as clean and nice as the porcelain; if you don't keep after
it, it rusts and really turns into a mess. All that notwithstanding, I
far prefer cooking on it over the porcelain... we've had probably five
or 6 porcelain grills on our old unit, and finally went for the cast
iron last summer... have never looked back.

Good luck!

eno