What are you using now? Do you have lights now you're trying to replace or other tanks that will be near this one?
5700k looks blue to some, yellow to others. 6700 looks white-bluish to all, unless you keep salt water.
If you keep the ratio at 50/50, you'll be around 6200K average, more blues and you'll be more blue. It's a preference.
I've had 5700Ks and thought they were blue, then swapped to 6700K and thought they were blue. Then added some 7800Ks and they were really blue (too much). Settled for a 50/50 mix of the 5500/5700K and 6700/6800K compact floros when I had a 4x55W AH Supply setup.