Depends on where your account is and the rest of your roster.

But in general I would say go with Kael, and take him to 6 with full masteries.

Mostly it comes down to books; a fully booked Kael is *probably* going to be more effective for you than an unbooked Big 'Un

Bigun, no question. Even unbooked, he's easily better. 

Fair enough. I don't have him myself, I bow to Kram's wisdom on this.

Big Un.  Big Un was my first lego.  He will replace Kael everywhere immediately, except for clan boss. 

Note:  you do not need to invest your books in Big Un to use him in campagin, dungeons, etc.  Just get him good attack, 100% crit rate & good crit damage, and he will carry you through a lot of PVE.  Can also be used as an arena Nuker in early arena, but falls off later.     

And then work on WuKong unless you need Kael for CB.  Neither Big Un or WuKong can replace Kael in CB.

Thanks for the info... and yeah I probably need Kael for CB.  Though right now my goal is to break through hard and get into brutal.  Yeah the accounts like... 6 days old... I got stupid lucky and pulled Big in like 5 ancients?  pulled a couple before this 2x weekend and regret that.  Big Un has just been a monster for me though when I saw that most people focus on Kael first and farmost I wanted to see if Big Un was a exception to the rule.  Well whatever starter you choose.

People generally focus on their starter rare because they need a reliable campaign farmer.  Big Un is one of the top 5 campaign farmers in the entire game, and offers great utility across many other parts of the game.