I cant tell you what to chose but i can tell you what i noe of both job's
Shillien Knight
This character is a "tank" character. Tanks are used mostly to go ahead and receive heavy damage while the rest of the group does the damage. They are known for their Cubics that assist them in battle. They can sustain lots of damage while the cubics help them in battle. They also have other usefull skills that make other players/monsters very weak.
In average i would rate them:
PvP: 8/10
PvE: 7/10
Bladedancer
This character is a good as a damage dealer and as a tank. Their main abilities are the Songs that are very useful when in a party. The songs they do can increase various stats making combat a lot easier. Although the songs dont last for long, their MP Regeneration is high and song MP cost quite low. Also, their Dual Weapon mostery makes them quite good in PvP.
In average i would rate them:
PvP: 7/10
PvE: 7/10
Shortly whats writen above
Shillien Knight is typical fighter char and is tanker which means he got lots of hp
BD on other hand is class i prefer over all classes ingame.He is good dd and he is suport char at same time. BD is very good char but only if you are ready to throw your MP away on dance's(buff's) to help party.
BD is equivalent in pvp to Gladiator(and if you dont belive i will post avi so you guys can see it

). Also BD when buffed have i tink highest critical chance in game

And i heared that some BD ard 55 lvl do 800 dmg on crit with samurai's double swords and thats i tink much more then Phantom Ranger can even dream on that lvl
All in all BD is pretty good job to chose
