As a BrM and WW I just don't like chi explosion... it feels odd. I do enjoy serenity though.
EDIT: I have changed my mind on Chi explosion.
First for wind walker....
I was looking at my damage and Blackout kick really isn't doing much anyway... so Serenity doesn't appear to be as good as I thought. I started focusing on using Chi explosion on 3 chi or more because of the TEB. In my current poor gear I can get 7-9 chi explosions off with 3 chi or more every 1.5minutes. The most I can get out of serenity is 10 BK/RSK which is 5 TEB.
So Chi explosion is more damage, more TEB, and better aoe but at the cost of being slightly harder to pull off correctly. This is very similar to the talent ferals where using at 90 to boost damage but at the cost of being harder to manage.
Now for Brewmaster...
Basically the same deal here. The focus is on only using Chi explosion at 3 chi but it is less punishing to use it at 4 when not needed. For brewmaster these talents are closer to situational. In the long run if you can use it at 3 chi explosion gets you more purifies/shuffle. So if you are going to take steady damage and/or need to AoE then Chi explosion is better. Serenity is better if there are short periods of burst damage with big downtime between (BK can be spammed while using purify and guard).