the 8-branch cross is the "Orthodox" one
the 8 pointed cross is the Russian one. The adoption in Russia of the heretical Greek 4 pointed cross was one of the arguments cited in favour of the Old Believer schism.
The Greek cross has arms of equal length, like a plus sign. It can be seen in both the current Greek flag (in the top left corner), and in the earlier post-independence national flag which was square, and simply a white Greek cross on a blue field. This flag still forms part of certain insignia, such as the presidential flag. The current national flag of Greece, the naval ensign, and that of the merchant navy are all now the same design.
The second cross that Michal posted is known as the Roman cross. The Old Believers got it wrong.