Quite often these people asking are beginners who are afraid of making the wrong choice. As with any rules of thumb, these are general and there are some exceptions to them. A few may be somewhat controversial and I am sure alternate views will be given by those with opposite views. However I intend these guidelines to point one in the right direction rather than providing a detailed map of what to do.
Any antenna is better than no antenna.
Rather than agonizing over an antenna choice, just put one up and operate. After operating with it for a while you will become aware of your operating habits and the shortcomings of the antenna you have erected. That will give you some hints as to which direction you should go with another antenna. I hear lots of S9 QRP stations. They would still make fine QSOs at S8.