> I thought Stephenson gauge was chosen because there were a lot more rail miles and rolling stock using that gauge than Brunel gauge.

That was key reason. The other was that 7ft gauge limits how tight your corners can be versus standard gauge. There are other minor disadvantages, such as that because trains are bigger, tunnels need to be wider.

The 7ft gauge trains were probably a lot more smooth though. And that extra ~3ft would make them feel much more spacious. Luxury was a big prestige element on the Great Western.