Can I just put in a good word for deferring? We're also in Scotland and we deferred entry for a year for our dd1 and ds, both first week in Feb birthdays. So they started school at 5y6m and we have been overwhelmingly delighted with the decision. They are both very bright and we could easily have sent them at 4 and a half but that extra year in nursery really allowed them to flourish and its been great for them. In fact, when dd2 started at 4y9m she seemed so little and she has really not done as well at school in the first couple of years - though there may be other reasons for that as well.
And because we are in Scotland there is none of the jumping a year (missing reception etc that you often read about on here) - they just stay in the year they are in, so my oldest are 10 in primary 5 and 8 in primary 3.
From experience, the smaller classes are much better, I would say. Personally, I don't think I would go for a catholic education as there can be a lot of emphasis on the religious side which we wouldn't want for our kids (though we are Christian, we just think that this is something for home/parents not school). Our school has classes of 25 tops and that has been good for the children too I think.