Yeah, I think we can sort something out for you, Cillian. What you're after is a jacket that you just don't have to worry about. If it's too much for certain conditions, fine. But you don't want to find it's not enough for others.
The Arc'teryx Alpha SV ($599 U.S.—no, that is not a typo) is certainly worthy of the title of World's Best Hard Shell Rain Jacket, if such a title existed. It uses Gore's very fine Pro Shell material to keep you dry and comfortable, then adds features such as watertight zippers, reinforced shoulders and arms, a helmet-ready hood, and much more. What's perhaps most amazing is that it tips the scales at 1 pound 3 ounces, which is pretty good.
The Theta AR ($475) is, in many ways, identical—similar details, similar reinforcements, same weight. It's cut a little longer than the Alpha. And it uses the somewhat older Gore-Tex material, not the Pro Shell. I'm a little puzzled by how Arc'Teryx manages the same-weight feat, but they do.
The Theta may be just a bit less breathable in some conditions, but generally would perform much as the Alpha. So it's a very hard choice. I'm not sure the Alpha justifies the fairly steep price premium. But if you want to say you have the world's best hard shell rain jacket, who's going to argue? Not me.