I am a gnomish archaeologist reaching LV14 at Gehennom with intrinsic telepathy, and I have to make a choice between Ring of warning and Ring of ESP.
Pros of Ring of warning:
Pros of Ring of ESP:
Which one is a better choice in this case?
In any case, given that you already have intrinsic telepathy, I wouldn't consider either of those items a particularly necessary part of an ascension kit. You're probably better off using your amulet slot for reflection or life saving and your ring slots e.g. for free action, slow digestion or stat increases. You might also prefer to reserve one ring slot for situational rings like conflict or levitation, so that you can quickly slip them on when needed without having to remove another ring first.
(Given that you're already in Gehennom, I'm assuming that you've found a towel or a blindfold by now to let you toggle your intrinsic telepathy at will, and have managed to make yourself see invisible permanently. If not, you might want to prioritize taking care of those issues.)
That said, both warning and ESP can certainly be convenient, if you happen to have them and a free slot. Just don't expect to be wearing them all game — you'll almost surely find a better use for those slots eventually.
If you happen to have both, you can pick whichever you personally like more. Or even wear both, if you want. In vanilla, I'd usually prefer warning myself, if only because I tend to have other uses for my amulet slot. Also, ESP is somewhat redundant to intrinsic telepathy; while warning provides less direct info, you can always put on your towel / blindfold for one turn to check what the monster you're being warned about actually is.
(The major exception, of course, is if you happen to be a high-level monk, caveman, healer, priest or wizard, or a post-quest rogue or valkyrie. Those roles receive warning intrinsically or from their quest artifact, making the ring redundant.)