Are you familiar with kiniseology testing of consciousness and other attributes? You can ask questions and get very powerful answers that reflect the truth and not the illusion. Read Power vs Force by David R. Hawkins for more on the potential on this.
Personally, I believe the answer rests within your make up. Each choice offers you valuable lessons for your life experience. But it depends on what experience, you believe, you need the most, as to what choice you feel is better. There is no wrong choice.
The way you have layed out your choices to the casual observer, seems to reflect the following:
Sharon: reliable, safe, devoted, dependable.
Lilly: spontaneous, challenging, idealistic, dreamer.
These two women offer very different archetypal makeups and thus supply you with different opportunities of growth in character and spirit.
Try this. If Sharon wasn't as reliable and was more apt to move on to another relationship, how would you react?
If Sharon was indeed cheating on you, how would you react?
Of the two, who do you feel best demonstrates unconditional love?
Who are you more physically attracted to (animal magnetism)?
If you are considering a family, which one is apt to br a better role model for how you see your children's growth?
Look at the situation from many perspectives and remember you can use kiniseology testing to get at hidden answers (which is why I mentioned it).