I'm leaving from Southern Utah to cycle through every country in Latin America in September or October. I don't know what rating of sleeping bag to get. I know Latin America is pretty broad in the temperatures it has but I want to get away with the lowest possible. I'm a warm sleeper, I get warm pretty easily in my bed in the summer. I have a Therm-a-Rest NeoAir XLite, nice and warm, I have been sleeping on it for 3 weeks. I sleep in a basement apartment and have been sleeping on the cold concrete floor to really test it out. I don't know if that will affect the rating of the sleeping bag but I thought I'd throw that out there.

How far south are you going and in what months? Chile and Argentina are thousands of miles north to south! I camped through a hard frost in central Argentina last July, and Argentina goes 2000 miles further south. How high are you going?
"Bolivia" could mean warm rain forest, or could mean sleeping at 12000 feet for days, before crossing a pass 2000 feet higher. You asked a question that cannot be answered without more information. What do you need to be comfortable in the US sleeping as high as you plan on going, in the season you plan on being there?

Re varying temps, indeed. You could find yourself at 0c in some highland in Mexico, 35c in Nicaragua and then -10c way the heck up high in Peru or something.
That's the one thing with this sort of trip, the range of night time temps will vary so much, clothes are one thing, ie you can buy an extra fleece sweater or whatever, but the sleeping bag issue is more tricky and you may end up having to compromise a bit so not be lugging around a -20c bag through central America and such before you need it.
Cgoab is always a good resource to see what others have taken on similar trips. Clearly having proper head, hand and foot gear is crucial to daily comfort while riding at higher altitudes, and I strongly suspect that having lots of layers to supplement the times of really cold nights is key.
Still comes down to it being tricky given the range of temps involved.

The xtherm is more expensive, but has almost 2x the Rs. maybe even a foam pad for back up. Did I say travel light, in the past? Or did I say safety first?

My advice would be to get a bag that is good up to 0-5°C (32-41°F) and combine it with a very warm liner like the STS Thermolite reactor extreme and layer or unlayer as you go. A sleeping bag that is too warm can be zipped open.

This way you have a system that will work between -10°C (14°F) and 25°C (77°F). Anything hotter and you probably won't appreciate laying under anyhting anyway, or bring a small cotton and/or silk liner for the tropics like this for example.

And bring thermals. Layers make all the difference.

I figured this would be an impossible question to answer. I don't plan on going farther south than Concepción Chile, and am planning on visiting every country and their capitals. I don't know the seasons sice I don't have it that planned out, I have a range of 18 to 24 months to complete this. I start throwing off blankets at 70° F and sleep with just sheets at 80°.

With such a broad range of temps should I get a moderate bag and bring a heavy liner or perhaps a quilt? Is it difficult to find new bags in Central America/South America? Maybe I could just get what I need for Mexico in October threw December and then upgrade/downgrade?

A bag liner is great for hot nights when you either lay on top of your bag or open it.
I only sleep inside my tent if it rains or there are way too many bugs, so the bag liner is essential to keep most mozzies away.
If it gets too cold (below 5C) I'll buy the lightest cheapest thinnest bag I can find at Walmarts or wherever and double up. That makes a HUGE difference.

What I'm trying to say is that in my experience there isn't a 'one bag does it all' from freezing nights to 30C nights.... especially if you sleep under the stars.

Some of those S. American capitals are between 3000 and 4000 meters. A 20F bag was survivable at 5000 meters in Ecuador on the flank of Chimborazo. One more layer would have been helpful or a VBL(Vapor Barrier) liner or clothes to extend the range of a bag or quilt for cold temps.

I did this trip with a 20F bag and the same liner mentioned earlier (thermolite reactor extreme). This was warm enough, barely, for the coldest nights I experienced (though I only had a 1/4" foam sleeping pad), and for the warmest nights I just used my sleeping bag as a pad, and still sweat through the night.

I was happy with my sleep system. I'd say 20F bag plus liner is a good way to go.