Best overclocking , best power section, highest power limit = Gigabyte, MSI, Zotac
Quiet work in 2D = ASUS, MSI, EVGA
EVGA, Palit, Gainward, Inno3d, standard version ( reference cooler ) of MSI, Zotac and some others = worse power section and max 225W on default 6+6 pin connectors.
ASUS has good power section but 1x8 power connector so also 225W max.
Gigabyte and MSI have improved power section and 6+8 power connectors = 300W max.
Zotac has improved power section in AMP/Exteme series and highest version has 8+8 power connector but it doesn't change much as these cards won't make much more than ~250-270W after overvoltage.
ASUS and all "reference" cards are clearly limited after overvoltage and clock is almost never constant under load. Usually on these cards max clock is ~50MHz lower.
Not like anyone would see any special difference if GTX970 is overclocked or not but these are the differences between these cards.
Somewhere there can be also Galax but I haven't seen their cards and I think I have no chance on a review sample ( if there is then please contact me
). Thie cards are almost not available in my area and if they do then prices are way too high.