First option: Poison damages 5% of someone's health each turn until they are able to remove the ailment manually.

Second Option: Poison damages 10% of someone's health each turn. But only last 5 turns.

As in which is better for gameplay?

Closer to a mixture of the two. Option A, but with the 10% damage. It makes poison worth doing something about.

It's funny because I actually made two bad stats like that. The first is called; "Poisoned", which does what the first option do. While the other is "Vemon", doing what the second option do. But if I have to choose one, it would just be the first option because I'm evil and like the player to remove it by item or magic. XD

I'd say go with both!

If we have both then imagine having them simultaneously on one target. Bit too powerful don't you think?

Well, if you're going to have the first option, then I recommend you make antidotes readily accessible; e.g. have one of the NPCs warn that "The forest to the north of town is filled with venomous monsters". If you want it to run out in 5 turns, then it's not as important to have antidotes on you.

Try something new, like go with A, leave it at 5%, but also have it so that poison also lowers the person's agility and strength. So while the poison is not as drastic as B (10%), it has other effects that may prove drastic. In battle, a weak warrior or a slow mage may turn the course of the fight over to the enemy.

No HP damage.
Your atk just gets weaker and weaker until your character is pretty much useless.

The first one.
You get nearly all the benefits of the second, with the added bonus that it doesn't go away.
The player can remove the status if he has the equipment, and if he doesn't, oh well. You get to see him/her waste away.
It's a win-win situation.

As others said, you could implement both options as separate statuses.

If you go with A, make sure to implement cheap ways to remove the status (items that you can buy almost anywhere for a small cost, and/or spells), and to warn the player beforehand.

If you go with B instead, make poison-curing items/spells more rare/expensive; this way, every time a character gets poisoned, the player will have to consider the situation and decide if it's best to cure him or to endure the damage.

Nah. Here's another fun thing you can do with stats. Make a status that has a 100% cure after round. The next turn, have them take something like 50% or 100% damage. Voila, you have a Demi spell. The worst part is that you don't really deal with the status, you deal with the massive damage.

Reminds me of pokemon except the second option the poison's damage (Toxic) doubles every turn. If you must choose one of them I'd say the first option because its more viable to use it against a boss, the second one would basically means you get half of that boss' health for free no matter what you do (as long as you can wait/hang on for 5 turns, most boss battles last more than 5 turns), so it may be a bit broken. It becomes viable if you make bosses immune to it, but I've personally always disliked how bosses makes status-effects skills useless and just take up space, status-effects should be a viable strategy IMO.

Sadly the first option makes it less desirable to use against random encounters, where fights are fast and the low damage poison would deal would be negligible, but I think its better than being overpowered.

I've always hated to manage conditions in RPGs like Baldur's Gate and the like. I think every DOT should expire eventually.

Sometimes, in the interests of the story, you could have a "plague" that needs to be cured by visiting someone (doctor?). If the plague is the central part of the story then it could be that it's hard to remove it once you catch it. Other than that, you better stick with a limited duration de-buffs.

Ever since I played Dungeons and Dragons, I've been far more of a fan for poisons which debilitate your stats. Taking 10% health damage means nothing, but slap some Strength / Dex / Con / Int / Wis / or even Cha damage and that stuff is suddenly scary.