arcane has damage reduction which farser doesnt have. However the damage reduction comes with the price of your prayer points. I wouldnt recommend using arcane at places such as nex where prayer drains quite fast and is essential to survive. Therefore, farseer is still good
It's good for tanking bosses, but I personally wouldn't use it for slayer. I've always liked to see people with it though, it shows dedication with dg as it's not usually the first thing you buy with the tokens.