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Okay, a couple things. You can go ahead and ignore captainmcd, unfortunately, because things haven't been that easy or that quick for a very long time. Also, you can't choose where you file for a marriage certificate. If you get married in Argentina, you get an Argentine marriage certificate. If you get married in the U.S., you get it there. Are you asking where you should get married? Also, to be clear: green card means permanent residence and permission to work and travel freely. Citizenship is a process that can be done later. Getting married does not get your spouse citizenship.
I'm not sure what you mean when you say that you might compromise your fiancé's chances are U.S. residency later on. Fortunately, the U.S. government doesn't care if the U.S. citizen (USC) half of the couple is or at one point was a transient illegal/irregular/permatourist/insert your favorite term here in another country. I was worried about this, too. (Just be sure you keep filing your taxes back home every year, even if your official income is $0.) You obtaining residency and a DNI in Argentina or any other country will certainly not harm his/her chances in the slightest!* That's not how it works. In fact, once upon a time couples living abroad could go through a process called Direct Consular Filing and get the foreign citizen a spouse visa to the U.S. in a matter of a couple months. This was only an option if the USC had legal residency, usually for at least six months, in the country where the couple resided. No need to send expensive packages back to the states to go through the whole process. Unfortunately, a couple years ago most consulates around the world stopped doing this. Here is the list of the current ones. Unfortunately, Argentina is not on it: http://www.uscis.gov/about-us/find-uscis-office/international-immigration-offices
If you don't plan to leave Argentina for two or three years, I'd say get married there, go ahead and get your DNI to make your life easier (well, maybe lol), carefully save and document all evidence of your married life together and down the line apply for the IR1 or CR1 spousal visa. Visa Journey is an outstanding forum, so have a look at this: http://www.visajourney.com/content/compare
You can also read the official stuff from USCIS here: http://www.uscis.gov/family/family-us-citizens/spouse/bringing-spouses-live-united-states-permanent-residents
One more VERY IMPORTANT thing. Does your SO have a tourist visa to the U.S. yet? If not, strongly consider getting one before marriage. Once they are married to a USC, foreigners are almost never approved, and without one, your spouse will not be able to travel to the U.S. at all until the spouse visa is approved. This can take a very long time. Or, if you change your mind and decide to stay in Argentina, your spouse might never be approved for a tourist visa, which means they can never make visits with you. Many people, I believe some forum members, have made this mistake. DO YOUR RESEARCH. It's not very romantic, but it will save your ass down the line.
*Depending on the type of visa you file for later on, you, the USC, might have to show intent to reestablish domicile in the U.S. This could prove complicated if you don't own property, have a car in your name there, have a job offer waiting, etc. So keep voting from abroad, maintain an active bank account, things like that. [/background]
