I think they are all good schools. I guess you have been to see them all? Have you worked out the logistics? Is one school easier to get to than the other?
My son goes to Dartford Grammar and he is very happy there. We are out of area, but his journey is direct and easy on the train. He also travels with friends and this is great for him. In spite of being from out of area, my son has made great friends and they often meet at Bluewater during the holidays. I think it depends on the child.
About the school itself, well, it is great. I like the fact that it is a truly international school were languages and science are very important. Dartford Grammar is one of few schools were ALL students are entered to take TWO languages at GCSE level. So, their statistics are not just great scores for the few who got entered for the exam. Many schools manipulate their results by not letting children sit exams they may either fail or not get the results they want. They are very professional, conscientious and very well organised. The teachers are excellent and treat the children well. Having said that, discipline is also enforced and at the beginning, the teachers appear to be very strict. They loosen up later!
There are lots of school clubs and the boys are encouraged to attend at least one. All clubs are free.
As well as in Beths, they test them regularly and give them grades for effort as well as attainment. Should your child's grades, behaviour or effort slip in any way, the teacher will phone you immediately! I really liked the personal touch that the headmaster reads and sings all reports. I was even more impressed to see a handwritten comment from the head to my son.
At the school they are very keen on rugby and all students in year seven must attend rugby practice on Saturdays for the first few months.
You may also know that, as well as Bexley, they do the International Baccalaureate.
We have friends at Bexley and Beths and they are very happy with the schools.
The other thing to consider is whether you'd like single sex or co-ed. Moreover, do you have any other children? If your son goes to a Bexley school, your other children would only need a pass to join the school. At Dartford, however, there is no sibling policy and if you are out of area, your other son would need to achieve a high score. This, of course, may not be an issue for you.
At the end of the day, you need to find what is best for your son. League tables and statistics do not mean anything as they do not predict how well your son will do or how happy he will be.
I don't know what else to tell you, but feel free to ask anything else. Whatever you decide, I think will be a good choice.