Re: what is better flat vs.round spring ?
Assuming that we are talking about coil compression/extension springs, Rhys is correct - round will typically be best (keep in mind that wound springs are really just torsion springs). Round springs are also relatively easy to manufacture, and avoidance of manufacturing defects is of huge importance with springs. Flat springs ( such as those found in Glock recoil spring assemblies) have the advantage of offering a relatively short compressed length and thus are often employed where space is at a premium.
Spring design, including the selection of material and manufacturing process is a whole career in itself. What are the desired spring rate and travel? What temperature will the spring be used? What is the desired cycle life? What are the packaging constraints? What are the requirements for corrosion resistance? Are there any especially important tolerances for spring rate, free length, solid length, etc.? How will the spring be held or captured in the assembly? What is the customer willing to pay?
There is no "better" or "best" spring design. Frankly, for many firearms, the problem is usually not all that challenging due to the modest cycle life and low operating temperatures - thus the use of simple wound round springs constructed from music wire or similar material.