I prefer box store brands, because at 9:30pm on a Saturday I can run to my local store and grab a repair part off the shelf. If a big box store gas valve or burner assembly (or thermostat or element for electric WHs) fits into an AO or Bradley, then I guess I'd have no brand preference.
As for 40 vs 50g... I've had both. For one person at a time showering, including two people right after the other, 40g has always done the job for us. However 40g just can't keep up when filling a bathtub (nor, I suspect, with those fancy showers with multiple body sprays) unless you crank the thermostat up way past dangerously scalding temperatures. So that's why I just put a 50g in.
As for 40 vs 50g... I've had both. For one person at a time showering, including two people right after the other, 40g has always done the job for us. However 40g just can't keep up when filling a bathtub (nor, I suspect, with those fancy showers with multiple body sprays) unless you crank the thermostat up way past dangerously scalding temperatures. So that's why I just put a 50g in.