mod_rewrite or mod_alias?

As the accepted answer says: mod_rewrite can do things which mod_alias can't. However the main benefit of mod_alias is that it is easier to use.

The apache docs say that we should use mod_alias for simple things like redirects and mod_rewrite only for things we cannot do with simpler modules like mod_alias. View the docs: When not to use mod_rewrite.

mod_alias is basically a simpler version of mod_rewrite. It can't do some things that mod_rewrite can, such as manipulate the query string. If you're able to choose either of them, I don't see any reason that you'd want to use mod_alias.

Is there a specific reason you need to try to use both together?

Apache mod_rewrite & mod_alias tricks you should know seems to be a good article about the two. It notes at one point that mod_rewrite rules get executed before mod_alias ones.