Going CO through Newark is probably the most effective as
1. You are always going in the right direction ! Nothing worse than leaving Scotland in the morning to London or Amsterdam, transferring, finally getting away, then finding that many hours later you are passing overhead Scotland !
2. CO have many nonstop flights from Newark to Houston. If you misconnect for any of the large range of reasons that cause this (and the Edinburgh flight arrives quite early in the day) they will just stick you on the next flight. They have many hundreds of misconnect pax each day there and are fully geared up for it. Conversely if you misconnected at Gatwick or Amsterdam you could be in for a 24 hour wait.
3. Although there is the customs/immigration procedure at Newark, you are just doing it there instead of at Houston. Arrive at IAH on a domestic flight and you're straight out.