I bought a MacbookPro and my wife bought the Macbook. We didn't buy the cheapest/slowest option in either case because that usually turns out to be false economy (you wouldn't buy a car model with the smallest engine and expect it to be fast and have good fuel economy would you? )
Anyway, the Macbook flies and back to back on the same wifi, it's impossible to tell which is faster. I have encryption turned on on both too which should slow them down but they are both really good machines. I'm sure that for intensive processing tasks the Macbook would be slower but for everyday use, these machines have way outpaced their requirements. Battery life is better on the Macbook.
I had serious worries before we bought it but it's an impressive machine and the form factor actually feels right. I really like it. She loves the fact she can slip it in a handbag and not have to carry a separate bag. Way too expensive but a superb machine.
The Macbook is a couple of months newer and the keyboard is slightly better. Maybe they've improved the manufacturing? It's slightly freer and more 'springy'. I believe they are the same mark 2 keyboard. I have no problems with either though.