We had the same dilemma last year!! DD was adamant on KEVIH, we live no more than 10 minutes walk from the SCGSG. Personally both myself and DH preferred KEVIH, but the distance was a major issue for us when we had a grammer on the doorstep.
She is not sporty so the new gym would not have enticed her, quite the opposite, as she would not want to run anymore than she needs too!!
One of our concerns was after school clubs. She has a slightly longer day at KEVIH than she would have had at SCGSG, but all clubs are during lunch and there are plenty to choose from. Both myself and DH work fulltime so do not have time to pick up after school, this works well for us. So far we have not found the distance to be an issue.
Basically, what i am trying to say is factor in after school activities, as extra-curricular activities are a major part of grammer school life.