Garen its really good 1vs1 because silence , shield and AoE with an ult who can kill a guy below 20% of hp , also he dont need mana so he can always spam (obv have got cd..)
Shen also its good in 1 vs 1 have a taunt that reduce 50% the dmg , a shield , can harass with Q and also the sustain , the ult from shen only go for an ally ( 850dmg in lvl 3 ) so he is good initiate and protecting in a TF , synergy champs , can jump over wall
both champs are really good, if you like to 1vs1 go for garen , if you want to fight in team go for shen
EDIT: I forgot passives .
garen : Great sustain can stand in the lane for a long time
Shen: Omg this harass and regain some energy to spam
I choose shen , i always like to fight in team