a. Russian oak is more premium grade, clear surface, less knots. less color variation. European Oak looks much rustic. So normally our AB grade is Russian oak and CD grade is European oak.
b. Russian oak has less tannic acid inside, and European oak has more, this will only influence if your color need to do fumigation, after fumigation, the appearance will be different. normally European oak looks better.
c. for unfinished floor, the grain looks slightly different.

Detail please check: