Is there a better side of the train to sit on for good photo opportunities? Thanks.
Either side is fine, except when the weather stinks both sides are horrible. I'd go as far as not making reservations because of the weather.
I completely agree with Ed. If possible, skip out on making reservations as they are not really necessary. Weather in Wales is a crap shoot at best, and if it is bad, the views make the journey basically worthless.