Itsalongstory,
A lot of people on this forum will have pieces of the information you want, but before you make potentially life-changing decisions about citizenship, taxation, and Social Security, you really ought to talk directly to legal experts and the Social Security Administration.
With that said, here are a few random thoughts:
--One potential advantage of citizenship you didn't mention is that LPRs can have their status taken away. Something as simple as a DUI could cause you to lose it.
--It is my understanding that you do not need to be a citizen to collect SS, and that you do not need to be a citizen or even an LPR to collect it. Again, you need to confirm this directly for your specific situaiton with SSA, not take the word of a stranger on the internet.
--If you are an American citizen or an LPR, your Social Security will count as taxable income, subject to normal IRS rules. If you are residing outside the US, you might be eligible for the Foreign Earned Income Exclusion, but SS won't count as foreign earnings.
--If your SS is not liable to US tax, you'll want to be sure of its tax status with your country of nationality. This differs from country to country, but most countries have an aversion to income that isn't taxed somewhere...
--BTW, one other thing to consider is the potential impact of retiring abroad or long periods of slow travel on your LPR status. It is entirely possible to lose your LPR status if you leave the US for extended periods of time. If this happens, you could get caught in a situation where it would be hard to come back to the US. You might not qualify for a visitor visa, and your spouse might have to file a new immigration petition for you if they viewed you as a returning resident rather than a visiting tourist. Again, I would recommend that you consult with a good immigration lawyer before you make any big decisions. This obviously wouldn't affect a US citizen.