Concentrate at max gives 26 att, and can only be used 3 minutes out of every 6 minutes. It isn't passive either, and takes place of an attack buff. Marksmen will probably be using warrior elixirs during regular training, so it's only an advantage of 14 watt for 3 minutes at a time, then the bowmaster probably uses their own warrior elixir or a cider for the other 3 minutes.
Snipe is the xbow skill that hits for max damage (currently 99999) whenever its activated. It kills non-boss monsters in one hit, and hits bosses for 99999 every 10 seconds (the skill description still says 15 seconds, but its really 10)at max.
The major difference in 4th job would probably be the Bowmaster's use of hurricane.
Also the storybook needed to start the quest so BowMasters and Marksmen can even get concentrate/snipe is pretty rare since only sharks drop it. In Broa, I've seen a few smegas of people offering to buy it for 80-100M mesos.