tl dr - Rain-X is better than carnauba wax.
I was reading an article by Wired Magazine about Rain-X. The article talks about how Rain-X works and is the basis for why it will (if applied properly) work better than carnauba wax in cool weather or in high heat.
Basically, the main ingredient in Rain-X which makes the water run off is called Polydimethylsiloxanes (PDMS). This substance creates a superhydrophobic surface on the glass which will repel water. The article states:
Polydimethylsiloxanes (PDMS) - A chain of repeating molecular subunits — two organic methyl groups attached to inorganic silicon-oxygen — forms a thin layer on the windshield, and the methyl groups provide low surface tension, which actively repels high-surface-tension fluids like water.
Rain-X also has chlorotrimethylsilane, which it states:
Residue from PDMS synthesis, this compound is regularly used to silanize — that is, chemically neutralize — laboratory glassware so organics won’t stick.
What is organic? Bugs. (As an aside: What's the last thing which goes through a bugs mind when it hits your windshield ... It's BUTT! ... Sorry, had to throw it in.) If this stuff will help keep organic matter from sticking to glassware, it will help keep bugs from sticking to your windshield.
The last thing which helps Rain-X is siloxanes and silicones, di-me, hydroxy-terminated, which the article states:
Fragments of PDMS, broken down by that sulfuric acid we mentioned. The hydroxyl groups help them bond strongly to glass, making it tougher for your wipers to disperse the Rain-X.
This will actually help the Rain-X stay on your windshield because it is bonding to the glass, even if you squirt alcohol based windshield wiper fluid onto it. Carnauba was cannot claim any of these things. It is an organic wax which repels water ... that's about it. There is nothing in it which will help it stay on your windshield when the wipers are going. There is nothing in it which will keep it from washing away when you throw some wiper fluid on it. Due to Rain-X's ability to stay on your windshield, it should outlast the wax in almost any situation.
There have been a couple of things which the blogs/forums will state are issues with Rain-X. One of those things is chitter ... where the wiper will skip across the windshield. I've found if you keep the wiper clean, this problem never occurs. The easiest way to accomplish this is when you are washing your windshield at the gas station, put the squeegee side on the glass under the wiper, the lay the wiper down onto pad side of the squeegee. Pull the squeegee out from under the wiper and see all the crap which comes off of the wiper. It will work a lot better as well. Also, usually when you start getting chitter, it's time to reapply the Rain-X. You can postpone this by using the Rain-X brand of washer fluid.
Another issue which may occur with Rain-X is after application, you may see swirl marks. There are two things you can do to alleviate this. First, only apply the one direction, such as horizontally. Second, you need to ensure you are following the directions correctly by using a wet applicator. Thirdly, use a clean damp cloth to wipe the are after its dry. A microfiber towel works wonders here.
Please note: This is not an advertisement for Rain-X. I do not work for Rain-X, but I do endorse their product (unfortunately for free). When used correctly, Rain-X works wonders. It absolutely works great at night when it's raining for being able to see what's going on around you. It will keep your windshield free of water, especially at speed.