I have one of each on my Welsh B, and love them both. Lengthwise, they're exactly the same, but the Shires is slightly deeper and hangs a little lower around his legs.
I have just bought a Tempest and very pleased with it. I would say slightly roomier round the shoulders and neck (my mare is not big shouldered at all but found Amigos a tiny bit cramped). Lengthwise the same.
Agree with everyone else - my welsh section a has an Amigo rain sheet and a Shires Tempest medium weight (well, did until she ripped the side off this winter!). Both are much the same length wise but the Shires is roomier round the chest and neck.