Hey gabrielpasv,
I would go with Apache2.
Everybody want as much speed as possible. So after hearing and reading about nginx I started out installing nginx on my test server hosted by DigitalOcean, but somehow couldn't get the configuration right. When trying to fix the rewrite problems I had for a couple of weeks, I started over using Apache. I just couldn't solve my problems while looking for solutions online, and I just got absolutely NO help on the forums. I decided that for me the time wasn't right to start with nginx, if there's so little people around to help out configuring the darn thing.
I have no experience with Amazone AWS but I can tell you that I like DigitalOcean a lot. When I made the choice to go with Apache, I had my application running in less than half an hour. You just choose where in the world your server should be installed, pick a distribution, click some features like backup or not and the server is done within a minute of time. Then you have to install the different servers like Apache and Mysql, and in my case I installed a control panel that did all that stuff for me. Of course you have a little configuration going on after the control panel was installed, but nothing special. And after copying may application to the server, I only had to change the base_url to get it running.
As for the speed, I wouldn't worry about that a whole lot. You have chosen CI to develop your application and CI is still one of the fastest frameworks out there, and you can look into web page and database caching to make your application run even faster. You can always put your application on a nginx server when your project hits 100.000 users a day or whatever...
Good luck!
