I'm wondering whether I should buy tryndamere or xin zhao. Which one in your opinion is more valuable in a team fight?

Well they're pretty different. Xin is more of a soft carry/anti-carry and Tryn is about as hard a carry as you get in this game. Either he crushes everyone or dies horribly without doing anything. I think Xin is a lot more forgiving and a lot less item dependent just in general. I also think Tryn really needs a dedicated rune page a lot more than Xin.

That said Xin probably isn't going to be soloing many teams 5v1, which Tryn can do if he's farmed up and they don't have too much CC. That's pretty rare, but if you want a hard melee carry that's Tryn.

Otherwise I think Xin is a better all around choice who fits into more teams much more easily.

tryn owns xin . hes the strongest char in game no1 can kill that m^tha f*cker trust me i have a tryn and hes FRIGGIN EPIC 2K DMG PER HIT AND INSANE LIFE STEAL HAHA TAKE THAT NOOB XIN HAHAHAHAHAHAHAH

next week (monday) xin should be free. if you can wait that long you can test him out. Other than that cant sum it up any better than what Teucer said.

Trynd's AoE is his spinning slash ability, which scales with AP. His only CC is a slow that only works if enemies have their back turned.

Xin's AoE ability is his ult, which does damage based on percent of max health. He also has a knockup for CC.

I'd say that Xin is a bit more of a team player than Trynd.

Word is Xin and Trynd are both slated for changes next patch, Xin possibly taking a heavy hit and Trynd possibly getting ult changes to compensate for the Exhaust changes. Might want to wait and see what happens.

Currently Tryndamere is a fun pubstomper, but he's extremely vulnerable to competent teams with Exhaust. Xin is a nightmare, he has pretty ridiculous burst damage, CC, and survivability.