I am not a teacher nor a native speaker.
I feel these two sentences have the different meaning.
When you are saying, 'I will complete the work ...', you will do it by yourself while 'I will have (get) the work completed (done) ...' has the meaning of you arranging for somebody else to do it for you.
1. Bill is cutting his hair (he is doing it by himself)
2. Bill is having his hair cut (a barber is cutting his hair)
I hope this will help.
I recommend you to wait for a teacher to answer to your question.
Yours sincerely, Alex.
Student or Learner