All the GTX970 is mainly because of its cooler capabilities.
Its very hard to explain in full details but will digress how it works.
Every brand has their own "special" cooler and they also have other type of coolers for a cheaper variant.
For example MSI GTX 970 has many variants, the Gaming Edition is spotted with a Twin Frozr 5 technology. On the other hand they have the GTX 970 Armor Edition, which is actually not as good as the Twin Frozr 5 but uses somewhat a older Twin Frozr 4 Advanced technology.
So it depends on the brands you gonna buy.
Zotac: Dual fans, OMEGA, AMP!
MSI: Armor, Twin Frozr 5, Tri Frozr, Golden Edition
Giga: 2x Windforce, 3x Windforce
and the list goes on.
DO NOT look at base clock speed. It does not matter which card you take, be it the fastest or the slowest. For Nvidia GPU Boost 2.0 it depends on the cooling capabilities of the card. The cooler it is, the better the performance. Also depending on the ASIC value of the card. The higher the ASIC, the boost clock will be higher, and thus you can't overclock that much. But you are somewhat already getting a very high clock speed usually the limit of the GPU. For lower ASIC, the boost clock will be lower, and you have higher room for overclocking, but you may not get the guaranteed high boost clock against the higher ASIC value.
So depending on your budget. See how much you willing to pay for the GTX970. This will determine on what card is suitable for you.