Agree with most comments here. The main thing is to not obsess over the swag, just choose one and get out there.
Most swags will leak somewhere in heavy downpours if the rain holds up for long enough. The original non-hoop swags are the worst, because the canvas wets out even if it doesn't leak, and it transfers the water to your bedding via capillary action, that's the same reason you should never touch the inside of a canvas tent when it's raining. I have slept in quite a few swags when visiting customers in the outback in my previous life, that's what got me hooked on swags for car camping. There is no such thing as a quick to setup, comfortable, warm, dry swag that is not also bulky.
Like simonm, I have an awning on the side of the 4x4 which can shelter the swag in a downpour. I have to say that so far, it has never happened. The biggest water threat in our areas is condensation, and the best cure for that is a domed swag which keeps the canvas off your bedding and allows a passage of mossie free air to keep it under control.
In the NT, the locals can tell how long people have been in the Territory based on the thickness of their swag mattress

There are different sizes of swags, but in the end the differences are not large. Swags are just bulky. Most doubles will fill the rear seat of a car, 20cm of width isn't going to make a big difference. They're better on the back of a ute or on a pack rack. When we're heading out with family, we take a double and a single Mitchell. Single goes in the back and double on the roof. They can be packed tighter if the bedding is removed, but then you have to pack that as well. Usually a zero sum game and more time setting and packing...
The worst thing about a swag? Packing it up, tying it onto the vehicle and then looking everywhere for the car keys. They're in the flippin swag!
