why jax or trynd?

Trynd has one set pattern. Hit stuff, stat check if you're hitting a champion, win hard or die trying based on that stat check. He's reliable as a split pusher, but unless you have a ton of experience as a melee carry player and the enemy team sucks at kiting and peeling, the only thing you'll be able to do is split push.

Jax is far more flexible in that way. While I wouldn't call his split push necessarily weaker, he has just a few more hurdles than Trynd that he needs to Leap Strike over to succeed. On the other hand however, his huge build diversity means he can itemize for the situation, and his play styles range pretty vast for a melee carry.

To rephrase, pick Trynd if you know all you want or need is a strong split push. Pick Jax for situations that require just a bit more depth.

Jax can build just bork and tank items and kill everyone... Balanced champ.

http://matchhistory.eune.leagueoflegends.com/en/#match-details/EUN1/1484559024/206126317?tab=overview

Trynd has sustain in lane, his dash isnt targeted and can become immortal.

Jax has mixed damage.

Thats about it. League has tons of champs that are basically the same thing.