"働くつもりです" carries a little stronger overtone than "働こうと思います" about someone's intention to work. "働くつもりです" have nuance of some specifically intention and plans to work even if there is not decision. (will) "働こうと思います" have nuance of only intention to work. Sometimes, There is not specifically plan. (hope)

働く＝to work, 働こう ＝ "let's" work So, 働くつもりです = I intend on working/to work, while 働こうと思います (this is not so normal to say) = I think I'll work/I think I'll "start" to work It's hard to give you an accurate translation since there's no context.