I've actually been at two very high achieving places - one also embraced fun and the other not so much! Now I personally totally agree with you and love to make learning fun, but not all schools have that ethos.
The answer to how the children got on is it depended on the child. The academic ones loved the challenge and the pace, as did the confident ones. (Since the aim wasn't just to get the higher levels up, but all children to achieve.) However there were always a few who found it a bit much and did get stressed, no matter how much they were reassured. (Not so much struggling, but feeling the pressure.) Do you feel your daughter would respond well to a challenge? Or would it upset her? Would you be prepared to support the school or would it upset you?
Also, did you feel there was fun in the high achieving place? Did the children look happy? Were there bright displays up on the walls? What does the Ofsted report say regarding how the children view their school? Is it possible they do nurture the children there too in order to get the best from them, or do you feel they are simply too pressurising? What is their ethos? (Some high achieving schools are lovely!)
And are you sure the rustic school is really nurturing and not just laidback and (as QuintEgg says!) making a tactical retreat from SATs? (Again, have a good look at what their Ofsted says, and watch out for the hints!)
One last thing to bear in mind is yes, middle-of-the-road - and even weaker - students can indeed be helped to excel with the right encouragement. But she may not excel even with more of a challenge and may have already reached her potential. (I know that probably sounds obvious but some parents can find that very upsetting if it happens, so I just thought I'd mention it!)
I hope that helps - please do let me know if there's anything else you need, I fully agree it's an important decision. 