Holmes, who took bronze over 800m at the Sydney Olympics four years ago, recently defeated European champion Jolanda Ceplak (pictured), raising expectations that she would opt for the shorter event.
But her heart has always been with the metric mile, despite the fact in two previous Games appearances, she has failed to make a strong medal challenge.
The 34-year-old realises time is no longer on her side and Athens will be her third and final Games appearance.
The former Army sergeant, whose 11-year athletics career has been badly affected by serious injuries, added: 'I'm not going to rush myself given this is my last Olympics.
'Obviously, it's a big decision and I've got to get it right.
'With the 1500m taking place after the 800m it's going to be a difficult one and I'm not taking any risks. There's a lot to consider.
'I'll be in a better position to decide what to do when I get out to the training camp Cyprus.
'I'll be training hard there and that will give me a good instinct, which I do have.
'I'll not be making any rash choices and I've still got three weeks to decide.'
