Lets's get straight to it.
We'll start with Klefki:
Prankster - Don't think this require much explanation. Priority status can be extremely annoying, especially Thunder Wave which can cripple frail .sweepers.
Dual screens - Utilises Prankster well.
Stats - Unlike Florges, Klefki has mediocre defenses and low Hp.
No reliable recovery - Klefki doesn't have any form of reliable recovery outside of rest, and with its defenses it will get worn down fast with just Leftovers.
Bad offensive movepool - Klefki's offensive movepool is practically one type in Fairy, which steel types can wall, and toxic doesn't affect them. Not that you would stay in.
Next is Florges:
Stats - Florges has decent Hp and very high special defense and good special attack.
Semi-reliable recovery - In the form of wish. You can run protect which also works well with toxic.
Support moves - Florges has access to Aromatherapy, which is a very useful move and can even win you the game in certain situations. It can also utilise wish to wish pass.
Bad offensive movepool - Just like Klefki, steel types painfully wall it. But again, you're obviously not going to stay in.
Bad defense - Florges has low defense. This can be fixed by running a defenskve set. Florges' special defense is already so high it doesn't need investment. It still gets 2HKO by powerful STAB moves though.
In the end, I would pick Florges a great special wall and support and just all around better in my opinion. It's your choice though. Although Sylveon can do everything Florges can better, Florges is the choice for UU.