I have two adult disabled children. I have guardianship over one and power of attorney with the other. They are very different people with very different disabilities.
My son is 26 and has Down syndrome and seems as if he is likely more like your son. I have his power of attorney. I would never consent to be is guardian. He is unable to make decisions independently and he understands this. So he seeks and takes my advice into strong consideration. He doesn't always make the same choice I would do, but he never makes a choice I am strongly against. A power of attorney preserves his rights as a independent adult - able to vote, enter into a contract, give blood, own a gun, marry, have children, etc. Not that he is necessarily going to do those things - but as adults those are our rights.
My 18 year old niece on the other hand has severe clinical depression, ADD, and is bipolar. When she recently attempted suicide, if it was her choice she would have come back home after wards as if nothing had happened. But because I am her guardian, I was able to place her voluntarily into an intensive residential psychiatric facility. And should I choose to take her out - it will be my choice. Versus had I had to force an involuntary commitment, which would not end when I wanted it to, but when the doctors wanted it to.
Simplified, the main difference to me between the two is that in the case of guardianship the rights of the adult are taken away. They are not up to the guardian to decide -they are gone. The guardian makes decisions the court allows them to make. (Such as how to spend money or which medical treatment to get.) So if you as a parent were OK with a decision your son made to enter into a sexual relationship with a girl, your son because of the guardianship would not be allowed to even if you agreed because he was no longer considered competent to make such decisions. And if the court or someone else dislike the way you were his guardian - his guardian could be changed. Just because you request it doesn't guarantee yo wil be the guardian now - or forever.
On the other hand with the power of attorney, you still have all the rights to make the same decisions you would as a guardian, but he loses none of his civil rights. It is essentially a contract between you and your son that he says he is willing to give you the responsibility to make specific decisions in particular events.
But the fundamental difference is - Guardianship is court ordered and only a court can reverse it. Power of attorney is an agreement between you and your son and can be terminated at will. (And you can pick up POA papers at Office Max for $19.95 and do it yourself.)
Until I came to parent my niece who is both cognitively impaired and mentally ill, I would have said no one ever needs a guardian, but she does, and I hope it is only for a few short years until she matures a bit more. I doubt very much from what you have said that your son needs a guardian.
Added: My son has a tested IQ of 43. He cannot read, do any math, or understand medical consequences. That does not make him incompetent to decide that he trusts me to help him make - or make - the decisions necessary for him to live his life well. Unless you and your son have an adversarial relationship - there is no reason for a guardianship. Whatever you can accomplish with a guardianship you can accomplish with a power of attorney. The difference is that you and your son maintain control and you have not given the courts control. When my son needs a medical procedure, we both listen while the doctor explains it. I then rephrase what I heard and say If you do A this will happen, if you do B this will happen. I think A is the best choice to make because of Q. Do you agree? He agrees. He maintains his independence. Or if he says, you decide Mom. He maintains his independence. And he maintains his choice to have you as his decision-maker.
Here's another example that might hit closer to home. What if the medical facility your son was in was not a place you felt he was safe. What if you decided that you wanted to bring him home and look for a better place. What if the medical facility petitioned the court and said that they felt his guardian was doing him a disservice (because you are now a guardian - and not a parent) and the guardian should be changed. And so the facility you disapproved of became his guardian. Now you have no ability to visit him, or have any say in how he is treated. And if you think it doesn't happen - sadly it does - and it happens far too often because parents thik they are protecting their child by becoming their guardians - when often they are doing exactly the reverse - they are throwing them to the wolves.