Hi K., This is my opinion, and I know it is not the current trend, but I would NOT send her to 6 hour pre-school. She is 3, and the time you have with her is so limited before she DOES go to school full day. Take the extra $ you would spend on the day care, open a college fund, and SAVE IT FOR THE FUTURE. They need time at home playing with toys, or going to the park, or baking with you or your Mom,.... This is not the time to be concerned with knowing your colors/numbers/letters. All that will come easily in the next couple years. NOW is the time to build her self-esteem by praising her art work, and crazy play dough creations.
My kids are all in school full time and doing great. They only went to 4 yr preschool for 3 hours/3 days-week and were NOT behind socially, emotionally, or academically when they started kindergarten.
That is my opinion--which you requested, but here is another thing to think about---
Only YOU know your situation, and you should ask yourself "why" do you think she needs more school? Is there a pre-school that is somewhere between 2 hours and 6 hours that would work for you?
Best wishes for you and your children.