In soloQ, i think Elise is far better.
Reksai really needs to make alot out of her early, and she needs to make best use of her global presence, which requires good teamwork, to be relevant.
Elise has overall superior 1v1, is better at taking objectives and scales better into lategame in more chaotic games, as with some ap/magicpen, even on the usually lower gold of a jungler, she can out duel nearly anyone and kill squishys within the stunduration.
If you got some friends at hand for premade action, go for reksai, she can certainly be a beast, her early-midgame impact in coordinated teams is massive, and if you trust in the rest of your team, beeing a tanky knockup bot at lategame is probably enough.
As for sustain, reksais drops a bit on the first clear, but after the first back, she's really well sustained, and her clearspeed also is rather high.
Elise overall has a really high sustain and speed in the jungle once you learned how to utilze her spiders for tanking without loosing them. Elise is certainly harder to play mechanically, but reksai has the more complicated decisionmakeing due to the options that her massive map-mobillity offers.
Both have really powerfull early ganks if used well.