The cores should be the same among all the 7850s and then among all the 7870s. The difference in clock speeds just means that it is factory overclocked ("factory OC'd"). These tend to cost more and may be binned better at factory among the brands known for OCing (like MSI or EVGA), but among the mainstream makes just means they adjusted clockspeed, something you can do for free with the MSI Afterburner option.
There may be some different flavors that have more VRAM as well, and cost more. Only select this option if you ever plan on going Crossfire (multiple GPUs of the same make/model) or multi-monitor/high-resolution (above 1920x1080).
Hope that helps, and don't hesitate to ask if you have more questions on the subject.