Many people have argued (pointless wars, trolling and such about it :/ ) in the past over which DSi site is better. I'll try keep an objective view in this blog.
*DsiPaint
Pros:
-Great Chatting System
-Bulletin Board
-Many admins
-Varied admin roles
-Good profile system
-Several games
-An MMO in development-TMJ
-Polls
-Paintings
-Web Hosting
-Friend code manager
Cons:
-Many admins
-Infrequent, but present, abuse of adminship
-Many alternate accounts

*DSiHub (assuming it gets back up)
Pros:
-Decent profile system
-Well made Points system
-Dedicated Reviews section
-Blogs
-Buzz, its news section
-QuickNotes, an arguably better painting program
-Forums
-Good commuity activities and events: Member of the Week, etc.
-An App store to expand its experiences: DSiCalc, Tap It, HubHub and more
-Hard working Admins & Mods
Cons:
-No Chatroom anymore, although being worked on
-Several inactive admins
-User abuse of HubHub as a Chat; against the rules
-Much Forum Thread spam
-Unorganized Blogs (albeit, there's a search option)
-Has been hit with hosting issues recently

*DSiCade
Pros:
-14 Games
-A Basic Chatroom: Shoutbox
-Users can submit games, if they created them and are DSi compatible
-A 1 for 1 trade off. You get a submitted game accepted and you can get a free DS game in return
-Several good mainstay members
-A News section
Cons:
-A few inactive admins, including the site owner (as far as I know)
-Users don't submit games these days
-Seldom Updated
-Rampant rule breaking when no admins are present, and even then sometimes
-Sometimes cloning
-Basic Chatroom

*DSiBook
Pros:
-A few, user-submitted, stories
-Decent Chatroom
-Decent "Coinz" system for use in buying avatars at its store
-A couple of games
Cons:
-The Chatroom page must be reloaded to see new messages
-Users never submit stories
-Some troublemakers plan raids there, at least once
-Rarely updated

*G/net&Tabs
Pros:
-Good Profile System
-Points System
-Allows for tabbing on the DSi Browser
-Blogs
-Most things, when loading, are set so that no page reloading is required to see them
-Basic Chatroom
-Status Updates
-Friend Code, Steam, & XBL Managers
Cons:
-Basic Chatroom
-Not a lot to do
-No real use (as far as I know) for its Points system

*DSiPlaza
Pros:
-e-Books
-Javascript extensions
-Reviews section
-"Apps"
-Themes
-3DSiPlaza, a 3D version of the site
-Users can submit many things for the site
-Groups
Cons:
-Only 1 game
-No Chatroom
-Few Reviews

Really, it's up to the user to decide where they like to go. Every site is unique and has its good qualities. Give them a chance.