In all honesty, I'm not sure if either really fits in well in Hydra. You don't usually want to run someone who isn't able to contribute anything offensively, and neither Rakka nor Elva place debuffs or do damage. While their defensive contribution is unquestionable, you often can't play people like them because of the limited space in a Hydra team.

As an example, my Nightmare team runs Mithrala, Maulie, and Uugo, all of whom are there for their defensive contributions - but all of which place invaluable debuffs as well. Conversely, while Wythir is an amazing champ for keeping the team alive, she simply cannot fit onto any team because she does zero offensively.

If you have to pick between those two for Hydra, then Elva will be better there, as she places SPD up on the team, along with placing perfect veil to allow someone to target Torment.

Personally, I'd probably try to put both Elva and Rakka onto arena teams. They may even work well on the same team - something like Mithrala + Elva + Rakka + Mortu, for example, could be really, really annoying to deal with.

I can't say anything about Elva, because I don't have her, but I regularly play Rakka in Hydra and occasionally in Live Arena.

Rakka definately holds her own in both areas, with providing a good amount of cleansing, healing, reviving and giving turnmeter boosts. In Arena she's definately inferior to Duchess or Pythion, because she's no damage mitigation, but especially in Live Arena when you're facing a heavy debuffer/bomb team, Rakka is very helpful.

Mine is build for Arena in a Protection Set with defense and support masteries. The Life Harvest blessing is a great fit for Arena.

i

I also play Rakka in my Hydra hard team, where she's more than use- and helpful, but I wouldn't put her on a higher stage in Hydra. 

i

All in all Rakka is a pretty good supporter for both Hydra and Arena, but she's not top tier.

Just from Live Arena (silver )

i