If you think your DC may need any sort of SEN support I would look at the SEN provision offered by each school irrespective of class sizes.
My DS is in a class of 29 in an oversubscribed two form entry school. The class has a teacher, TA and then there is another TA who looks after one child with SEN. The group of children needing extra help are also taken out for extra literacy etc with another TA so the staffing levels seem better than one might expect.
However surely it is not just about the staff/child ratio but the quality of the provision. If possible speak to parents already at the schools you are looking at to find out if the brighest children are stretched and whether those who need extra help eg. those with dyslexia are given the right support.
I have experienced driving one of my DCs to a school a few miles away and am moving them to a local school. The driving is time consuming and costly....at least £150 in petrol a month plus wear and tear on the car, a lot of stress sitting in traffic queues, negotiating pot-holes and 4x4 drivers on country lanes etc! Not to mention the environmental impact. If you use the school coach it would still need to be paid for and you would not have daily contact with your childs teacher which is particularly important in the early years.
Your child may end up making friends with people who live 5 miles the other side of the school meaning driving miles for parties etc.
The cost of petrol is only going to go up!
When I was at primary school I went to a rural primary a few miles away. However free coach travel was provided and in those days there was a lot less interaction between parents and the school (my parents only turned up at school for sports day and parents evenings) and no after school clubs.
Do other people in your area use the school 5 miles away? If not, surely the local schools must be good enough.