I love the Hydra, but the bug gets on my nerves every single time I see Blue mana open. If my opponent were fair dudes and let the ability resolve before countering it, but no, they have to exploit Stainless lack of patches.
/rant
(And yes, I am aware that most people don't know about the bug)
I'm not a big fan of Rescue in decks not made to support it. I hear people saying how awesome it is to tutor for it with RSD (I think it's you, not sure), but it feels meh to me. What if your 'yard just has small dudes? Are you going to do it just to recycle RSD's EtB? Why didn't you get another threat in the first place? I think the card can go either way. I probably wouldn't run it.
I love the concept of Prowess, even though you have to be very careful with the card. Problem is... It feels like win more on a fattie. I'm not sure how often do you have a weenie hanging around, and how often that happens and you're not facing a barrage of disposable creatures to mitigate your card draw. It's good, but situational. Would probably try a singleton.
What are the matchups that you need to draw more cards? Control decks packed with removal? Have you tried Triumph there?
To be fair, the control players who counter the Hydra would counter it anyway; it's not the same as killing a Bushwhacker with the trigger on the stack, it's making a play that you would have made anyway. but I agree that it sucks.
I understand the hesitation on Rescue, but with the Wayfinders and so many EtB creatures it has a lot of value here. Like I said, though, I'm certainly open to just a single copy. The allure of tutoring through RSD is that if you untap with the Demon then you can sacrifice it to grab a card from your 'yard and another from your library, essentially doubling its tutoring power.
Prowess is mainly on my radar as a finisher, but I haven't needed it; I think I'd rather have a Hydra, despite how poorly it interacts with Rescue. Another bomb (Hydra) is probably better than a win-more card that requires a bomb to operate (Prowess).