Single pump or double pumps better?

[TANGOMAN]

Do you have spec's. on flow in comparison to height(head) ?
I'd be curious if the mag 9 head is equal to 1/2 of the head spec. of a mag 18. If not, I'd be inclined to go with a single pump. A benefit to two pumps is, if one fails, there's still one running. Dual pumps should be plumbed with individual returns.
Is this a basement sump arrangement ? Are the Mag 18 flow rates going to give ya' a good "turnover rate"? I like my mag 12. It is noisier than the "Quiet One" I have running on a different tank but the flow rates are obviously less than the mag 18 and probably insufficient for your needs.
It may be more "economical" to go with a single, "beefier" pump...