Multiple custom permalinks structures or which is best?

I’m in the process of re-designing a site that currently resides on two WP installs. One install is an old blog (I will refer to it as “old blog”), the other functions as a CMS (I will refer to it as “old site”).

I’m developing the new site (I will refer to it as “new site”) locally. My goal is to combine the old blog and old site into one new site. I exported the old site and imported it locally on a fresh WP install. I haven’t migrated the old blog yet.

I need help formulating a good plan for combining the two and structuring the permalinks for the new sitesite.

Permalinks Questions:

The old site’s permalink structure is: /%category%/%postname% Which is a poor structure.

The old blog’s permalink structure is: /%year%/%monthnum%/%postname% Which is good for performance.

I would like to make the new site permalinks either: /%year%/%monthnum%/%postname% OR /%year%/%postname%

This works well for the blog posts. However, the old site also has a ton of posts in an “Articles” category. The Articles posts should have a more SEO-friendly permalink structure. Hence why the last developer used category in the structure: articles/postname.

Is there a way to make two custom permalinks structures? Keep the year/month/postname as the default, then add one for the Articles category?

I’ve heard about the Advanced Permalinks plugin. Does anyone have experience with it and is that the way to go?

I’ve also heard that in the fall WP will come out with an update in which %postname% will be okay for performance. Is this the best structure for all posts?

Then there are all the redirect issues.