I'm in exactly the same situation!
We have seen 2 schools, one is very high achieving with most pupils gaining 5 A* - C including maths and English at GCSE. it is an academy and very well presented. Parents are not encouraged as much to be involved with school, discipline is tight. A great place to learn. The visit felt very stage managed but we got the feeling our ds would achieve without doubt.
The other is a specialist sports college, extremely good on pastoral care, not as well presented (buildings are scruffy). The Head came across as very much wanting to involve parents, the home school link worker was there, and it was open house with the school being thrown open with the high achieving pupils showing us around. Very friendly.
First of all, go by your gut feeling.
Our ds has said a couple of times he would like to go to university - at the moment he is average in maths and science but very strong in literacy.
I think the first school would be better for him, but he is very anxious! The second school is very good at pastoral care and when I asked about the transition they said nothing is too much trouble, he could have as many visits as possible, etc.
It's just a difficult decision 