Yes the powercolor will overclock aswel, i also owned 2 powercolor 7870 ghz and they were great quality, get the 7970, don't downgrade the card itself for a better brand
If you want faster gaming at the expense of noise (the 7970 will be louder) get the 7970. The sapphire 7950 should be able to overclock higher though so you may be able to match the 7970. If it was me, I would get the 7970 simply because it's the better card. Also I've owned powercolor before I bought their 7870 XT it was good quality
Yes the powercolor will overclock aswel, i also owned 2 powercolor 7870 ghz and they were great quality, get the 7970, don't downgrade the card itself for a better brand