No one answer. It depends on the rest of the suspension and what the car will be used for.
Normal use, pick a spring with an arch that sits near flat at normal load.
Drag strip use with slicks, a different story - may want arch, definately will want a very stiff front half of the spring.
Don't go stiffer on the rear spring rates without going stiffer on the front anti-roll unless there is a specific reason.
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is my ride height lower than stock?
including link to a source for stock replacements to match an original arrangement.
