When the GL's first came out, SLKR was clearly better in PVE areas (soloing the Sith Raid for example) and Rey was clearly better in PVP areas. Seemed like a fair trade off. They both were much better than any other character and each was the best in a certain area of the game.
Then they buffed SLKR and now he is way better in both areas. Rey is not that good in PVE areas, yes she can 4/4 in the light-side GeoTB's, but not always depending on RNG, she is not needed for any of the special missions in the GeoTB. Is is also not great in the Sith raid (and sadly there is no other PVE environment to use her)
Add in the fact that she is now much weaker than SLKR in PVP, yes I know it is possible to beat Kylo with her, but it is not easy, it just seems dumb to go for Rey unless you are really trying to get both. Why should someone go for her when SLKR is much better in every area of the game.
Bottom line - if you are only going for one GL Kylo seems like the much better choice.