It depends on many things.
Here are some advantages of going with X58 chipset
1_6-core CPU support
2_Better OC'ng capability
3_dual 16x SLI/CF(which doesn't have much of a difference with dual 8x mode of P55)
The only advantage that a P55 build has,is that it offers similar performance(although i7 930 is a little faster here) and costs lower.