Which one would you rather eat? You of course have the option to decline both. It's a false choice to claim you need to pick either.

Do you, though?

I mean, that's just a construct of your hypothetical. But if all your choices involve sh1t as an ingredient, then you have to figure out which is the best of those alternatives.

Iran is a large, sovereign country with significant economic assets (oil) and strategically advantageous geography. That constrains our choices in trying to get them to do things we want, or force them to not do things we don't want. We don't have an unlimited set of choices.

We don't want them to have nuclear weapons. They have, of course, agreed several times that they will not have nuclear weapons, including but not limited to entering the formally binding Nuclear Non-Proliferation Treaty that says the will not have nuclear weapons. However, we do not trust them to honor those obligations.

That means we have only a few options:

1) Enter into some sort of agreement with them that gives us sufficient monitoring, access, and enforcement powers that we deem it a non-trivial restriction on their ability to get a nuclear weapon that is better than leaving them unconstrained - offering to them economic inducements (including relief from existing sanctions) to get them to agree; or

2) Keep all of the existing sanctions and eschew any agreement, thereby leaving Iran with whatever constraints might be imposed by such sanctions but otherwise free of either constraint or monitoring, to engage in whatever they might choose to do; or

3) Invade with ground troops and overthrow the government.

It sure sounds like you're claiming that all of the possible deals under basket 1) are ass, or sh1t sandwiches. There's absolutely no reason to believe that anyone could ever get a better agreement out of Iran than Obama did. So either the Obama agreement is not ass, or they're all ass.

We know, though, that #2 is completely unacceptable. We saw what happened with it. Iran was under sanctions but not under a deal, and (per the administration) they got within two weeks of a nuke. So that's also a sh1t sandwich.

That only leaves #3. Which, after our experience in Iraq and Afghanistan (two much smaller countries), we know is a sh1t sandwich with a sh1t topping and extra sh1t on the side.

So - what's your choice? You don't get to not choose. Iran exists, they have their own sovereignty and agency, and you have to do something in response to their decisions. You don't have the option to "decline" Iran having a nuclear weapon just because that's what you might prefer. You have to do something, and none of the choices are palatable.