I am so torn between jungle Graves and jungle Rek'Sai

Well this is sad.

Rek'Sai is the much better pick for jungling. The mobility alone allows you to impact the entire map. Plus Rek has everything you could want in a jungler, good damage, tanky, escapes, CC, sustain, poke, and ability to see through the fog of war.

Graves only really brings damage and some tankyness.

I prefer graves over rek'sai, rek'sai has literally everything you could ask a jungler to have but because of that she feels really mediocre in a way. A master of nothing.

They're both LCS staples so they're always looking to get nerfed tho