Baseline I'd personally rather deaths advance for PVE over steed simply for the cooldown of 45 seconds vs 60 seconds. For PVE 30% is enough to get out of anything, and the cooldown difference is pretty significant. In legion I would argue that even while double steed only really allows a single extra use of steed per fight Ret's pulled ahead in PVE mobility with steed on that 45 second cd.
In BFA being able to talent into a second CD where the cooldowns are separate makes DKs far more mobile than ret. Once BFA hits DK is ahead baseline and with mobility talents.
As sad as it is Ret will be the least mobile spec in BFA due to mobility nerfs, and when you throw the nerfs to defensives as well it's fairly disappointing. Just hope after tuning our damage is high enough to compensate, I don't really want to raid on anything else.