Sadly Smartpak only has limited weights/sizes. Horsebyhorse has a larger selection and is also stateside. So far PE customer service has been helpful and responsive to questions.
My 16h friesian-shaped boy is 81" in Rambo, but the PE 81" was really long on him so the 78" was right. I would say both are similarly snug at the shoulder on him (though less snug than the Schneiders). PE is slightly roomier overall with the gussets. Both stay in place better than the Schneiders, which slides back sometimes and generally bothers me more than him.