Thanks for your rep.
since I done some research about this.
I think I better go with separated site. (shared DB and WordPress file)
It would generate just about 0.5MB extra data in DB.
But this just my personal reason
Wordpress Multisite design for community blog
Despite it have a domain mapping plug-in, It shared a lot of thing on each site.
Since the wp-config.php is editable (mentioned on official site)
I think it wont cause a problem with my tweak method.
Thanks again.
PS.
I don’t conclude anything about resource usage.
(Some of hosting company in my country banned WordPress MU.)