You had it right, some of the copy on the EA site describes it well...
"Customization is making your Javelin play like you want..."
"Personalization is making it look like you want..."
Customization has some impact on your visual look as models change based on what you have equipped...
The personalization system lets you can add colour, vinyls/decals and meshes (armor packs) to pretty completely modify how the suit looks (as you've seen in some of the twitter posts and videos) 😊.