I would base the decision on what motherboard you want and whether you will SLI or CrossFire two GPUs. The P55 motherboards are cheaper than the X58 ones, unless you want to CF or SLI, in which case you will need a P55 mobo that has 16x/8x PCIe 2.0 slots. Such P55 mobos start around $160. You can get a decent X58 mobo for around $200 (like the Asus Sabrecat).
Another consideration is how much RAM you want. P55 mobos support 4GB or 8GB, and X58 mobos support 6GB or 12GB. if 4GB is enough for you, they cost around $50 less than 6GB kits.
In general, I have found that the price difference between an 870 and 950 system to be around $100.
Thankyou for your reply, I do not plan to SLI, to me is more important frequency and speed because for games, so I think made the choice), the more mobo is cheaper.