I guess it actually depends on what you are planning to do with the card. If it's for gaming on a single screen the core clock speed would be the most important. If you like to run high anti aliasing or more than one screen the memory clock could be an important factor.
The 7870 is a good card for overclocking (in most cases) so I think I would personally buy the card with the 925 core clock speed and overclock it to (hopefully) >1000 MHz. 1500 memory clock speed is actually pretty high for this card.
Do you have a link to the cards?