as said before the syntax and the antidogma are practically the same ski. the syntax is just lighter due to lack of material . if you need a more stable ski i would go with the antidogma.
if i were you i would go with the 172. im 6"1 and own the 178 ones (bought them when i was a bit smaller (like 5"8ish) and the size was and still is good for me.
hope this helps. you will never regret this decision. antidogmas are the best skis i ever rode (even if the topsheet chipping is enourmous, which i said before)
light, playful, good stiffness and good pop.
P.S if you kill urban or do a lot of rails (i mean really a lot) you might get edge problems. some amplid skis had issues with the edges and they ripped out. but amplid has a good warranty service so if this problem occurs (which it hopefully will not) you can contact them and the see what they can do.
fortunately i did not had this problem on mine, even if i do a lot of rails.
hope i helped you deciding. keep us updated or write a review if you are or are not content with the skis.