Chansey
Answers
ZH counters Machamp ( a popular attacker into Blissey) and HB is a strong STAB move
ZH: non STAB + Super Effective vs Fight
+25% vs. Machamp and Poliwrath
Pound: STAB + not Super Effective vs. Fight
+25% vs. all except ghost / rock / steel
ZH is slower, and so it is the better move for defending.
---but---
It you say, it counters Machamp, you could also say, that Pound counters every single attacker except rock-, steel- or ghost-types.
I guess the point is that most of us are switching to using fighting types to counter or prestige against Blissey (high CP Machamp for attack, Machoke & Primeape plus lower CP machamp for prestige). Thus in true defense role, ZH is better: it does 1.7x the damage per round compared with Pound, despite lack of STAB. And because Blissey's attack stat is low, the lower damage fast move will suffer more from the Floor operation in the damage mechanics (which rounds down damage to nearest integer).
That said, I personally find hyperbeam so easy to dodge that if I had the candy and a second candidate (like the OP does) I would evolve the second Chansey: Its already at level 27. Since there is no truly bad moveset, they will both be useful defenders and 2 is most definitely better than 1!
Looks like you got a very good Blissey, congrats! Zen-DG is obviously the best moveset, but Zen-HB isn't that much worse. Even a dodged Hyper Beam does 47 damage w/ STAB. Really any moveset without Psychic is borderline top-tier, even including P/HB.
Beyond that, I think ZH-BS Snorlax is probably more annoying to fight than a Psychic Blissey, but all other movesets are a tougher go than any Snorlax. All this is to say I would definitely power up this Blissey. It's quite good.
2 is always better than 1. I dream of having 2 with such good IVs. That second one is already going to evolve to CP 2480 or so and be a useful defender straight away even with no powerup, regardless of the moves it gets. And your first one already has a decent moveset by any definition, so without any further walking you already have enough candy to power that one up right away to level 31.5 (CP~2814, if we assume its presently at level 24).
So worst case is you end up with 2 Blisseys, one at ~2480 and one at 2814. Lucky you is all I can say!
If you evolve another, then you are committing to having Chansey/Blissey as your buddy in the future, just to maintain it.
No matter the moveset, it is still a better defender than any other mon in the game.
A better moveset will not earn you any more pokecoins or stardust than the one you have.
You only have a 50/50 chance to even get ZH, on the new one.
Just power up what you have got.
Of course, if he is currently only at trainer level 30, he can't power it up past 31.5 anyway and he has 109 candy already. So I say he is better off taking the punt on a second Blissey, and maybe getting better moves still and powering up whichever is better. The 59 spare candy after evolution is still enough to even get the less powered up one to level 31.5. Both will then still have potential to have very high CP if he walks one as a buddy while already using the other in the gym straight away.
He will level, and will want to power up again in the future.
What exactly will he gain with a better moveset?
Is it suddenly going to scare attackers away in a manner that his current one doesn't?
I can not see what the benefit of a better moveset on a lower CP mon will achieve...but maybe I am missing something.
The odds are not in his favour...in fact they are stacked heavily against him getting a better moveset.
Take it from someone who evolve 3 Zh/Psychic Blissey in a row - even though I spread the 'evolves' over 3 days.
If he wants to maintain 2 Blisseys with walking a buddy, then sure go ahead - but if that was the case he wouldn't be asking the question.
I dont think either decision is bad but each decision will result in you having to play a certain way. Do you need one bigger defender? Or if you battle a lot you can leave behind blisseys for your opponents to deal with. Are you walking a Dratini/Larvitar now and are unwilling to switch to a Chancey? Your gonna have to decide the game you wanna play but I dont think either decision is bad. Just make sure you decide whats good for you not whats good for the player giving you potentially bad advice.
I have 5000 battles and 1200 trains under my belt. I obviously like to fight. Im personally blessed to have 2 P/DG blisseys(level 37.5 and 30) that I leave behind for my local spoofer to deal with. I love having this option and drop them off only at local gyms on a daily basis. So annoying lmao...