I started Western and switched to English, so right now I'd tell you English is WAY harder. Now my 6yr old started with English and dang, she's up there in the perfect position, making it look sooooo easy but then she didn't spend a ton of time riding around in a Western saddle which forces your legs way farther forward than you want while riding English. She started off being placed in the proper position and having the instructor reset her position every time she lost it.
Age has an affect also, I'm not *old* but my muscles and joints do not respond like they did 20 years ago! I wish they did but no......
For the moment let us forget the technicalities and look at what Delfina as written as the practicalities. She has hit the on the key issues.
Delfina came to ride Western and that is what her body and brain has got used to. Then in later life she switched to English Dressage - which is a very 'formal' way of riding. As an older person she found this transition difficult.
Yes, because her brain had to reeducated and different muscles had to be built up.
But her 6 yo daughter gets to be trained English as a youngster and her English seat is taught and implanted in her from the very beginning. The daughter finds it easy and she looks good from a early stage. In English dressage everything goes back to the correct seat position.
But it will be interesting to see how she might ride Western if introduced to it in later life. My guess is she might not like riding one handed with loose reins and leaning over. Although if she is a natural rider who found horses at a young age maybe she will take to Western like a duck to water.
Personally if I had known what I know now then I would have learned to ride in either Portugal or Spain where the traditional system has similarities to both Classical Dressage and what we call Western. But I came to riding late in life and sadly and in all honesty I have never been an elegant rider in any system. Somehow I get by, either in an English or a Western saddle.
There is more than one system of riding and we should all experiment from time to time, whenever we get the opportunity. But no one system is better, each system is different and has been devised for a different purpose.