For first question, congrats. I'm a graduating MD/PhD and I regret working that first summer on science. Go have fun. It will be one of the last times in your life when you have no obligations. Working on a grant is a waste of time; you don't have any preliminary data, you probably don't have a solid mentor, you won't be working on it for two years, and filling out the face page takes like an hour at most. Don't study for med school. You will have plenty of time to do that later.
With regards to choosing a program, I think there are three things to consider
1) Does the medical school and graduate school "feel" right? Are these people you can see as your future peers? Do they seem fun? If you left the program thinking that everyone you met was a jerk or simply just got a blah feel, it's probably not right for you. If you can, ask around quietly about people who dropped out of the program. Some programs have severe attrition rates which may shine light on hidden problems.
2) Are there a plethora of quality mentors to choose from in yours and related fields. I think a lot of people fall into the trap of identifying one mentor and choosing a school because of that. That is foolish. Mentors move. Their labs fill up. Good scientists make terrible mentors. There are (often) unforeseen personality conflicts. Your interests change. Go to a place that has a lot to offer. Remember, you do not take your PhD project with you once you graduate, so you are there to learn to be a scientist. Your project is secondary to the mentorship you receive.
3) What is the location like? This is far more important than people make it out to be. You start the program at 21. You finish at 30. A lot in life can happen during that time personally. You may start single and finish married with two kids. Think about the dating possibilities in the city. If you're not interested in that, remember that the city has to offer you what you love doing. If your passion is climbing mountains, then Houston is not going to be a good fit no matter how good the program. Of course, if your passion is WoW, you can go anywhere...
Good luck.