Having personally done the Grind for Grimoire for both Hydra and Ogre, I can tell you this:
Ogres are more rare, there are a total of 3 that spawn in patrols. 2 on Cosmodrome (Skywatch and Grottos) and 1 on the Moon (entrance to Temple of Crota).
The fastest Ogre cycle is 36-37 seconds seen here. It takes place in the Shrine of Oryx mission.
The fastest Hydra "cycle" is right outside the gate to the black garden. There are plenty of Hydra that spawn in Vex missions, but a lot of them are either Majors/Ultras (and those DON'T COUNT!) or they aren't close enough to a checkpoint to have a good cycle. The Black Garden gate cycle can be done in about 55-65 seconds. The problem here is that the Hydra spawns with 2 Hobos and 4 Minotaurs. You have to kill all of these enemies to trigger the respawn cooldown.
There are also possible delays for the Black Garden gate if any nearby player actually wants to use it for it's intended purpose. It causes a hiccup in the respawn. Similarly, the visual haze effect of opening the portal can be an issue as well.
TL;DR Approx 90 Ogres/hour vs 50 Hydra/hour
Thanks for the tips, but... where the hell is there an ogre in Skywatch on patrol?
EDIT: WEEEEEEEE!!!
http://i.imgur.com/vG2litZ.gifv