A-TNF drugs such as humira and remicade are considered big guns, because they were orginally designed treat advanced sign/symtoms of crohns (such as fistulazing, blood loss, etc.), but more recently the thought has changed to let's get this under control before the disease gets to that advanced state. Because these are generally drugs that take a couple weeks to months to be effective.
Cimzia is the newest drug out of the "big gun" category, but all them are still rather recent.
Immuran and 6mp from my understanding are considered "safer" in part because they have been around longer and we know what their long term effects are. Futhermore there is a slight more risk in taking an injectable or an iv.
Endocourt is also consider by some as a long term treatment drug.