What's the difference? Which is better to use? Which is quicker? Any ideas?

I'd recommend using mod_ssl. It takes advantage of Apache's ability to load modules dynamically. This means you will be able to load the mod_ssl.so without having to recompile Apache. Also, if mod_ssl is ever updated, you do not need to recompile Apache. The other option is to compile Apache with SSL support built in. Could be a pain to do. I'd go with the module.