Ok...so here goes...
I have owned a Universal Bosch since I was married over 6 years ago, and I love it, but I have always wondered what it would be like to use a KitchenAid mixer. I talked to my mom about it last year and she said that she once bought a Kitchen Aid because it just looked so nice, and everyone (meaning, Martha Stewart and Rachel Ray) were using one. After using it for a little while and especially for making bread, she returned it because it just didn't measure up to her Bosch.
Yesterday, I had the opportunity to use a KitchenAid Mixer for making bread. There are several things I didn't like about it. 1. I don't like how the mixer arm is right in the middle of the bowl, therefore getting in the way of whatever you want to do. ie, adding ingredients or feeling for the consistency of the dough. 2. I was using the flat beater attachment (which I think was the wrong attachment for mixing dough) and as the ingredients were being worked into a dough, it all stuck to the attachment and just rolled around the bowl instead of actually kneading it. I must admit that I was making a 100% whole wheat bread and so the dough was heaver than with other kinds of bread, but my Bosch doesn't have a problem with kneading at all!!! I can knead 7 lbs of Whole Wheat dough in my Bosch and it doesn't blink an eye. I know I probably am not comparing apples to apples, but below is directly from the Pleasant Hill Grain Company website. Makers of the Universal Bosch. http://www.pleasanthillgrain.com/bosch_universal.aspx
The Bosch Mixer has More Power: For starters, the Bosch Mixer has a 700 watt motor. Our biggest competitor (the sales-volume leader) calls their strongest model the "Professional." Its motor is only 525 watts. Universal's motor is more than 1.3X stronger than their best. Power to mix better and last longer.
The Bosch Mixer has a Stronger Transmission: Even more important than the motor is the transmission, which converts motor power into work. Our mass-marketing competitors build transmissions with inexpensive spur gears. The first shortcoming of spur gears is that they break under stress. Stress is a bowl full of bread dough or a triple batch of oatmeal chocolate chip cookies. Things you need your mixer to handle. Buyers tell us frequently that they're getting a Bosch Mixer now because their other mixer, often brand new, failed.
The Bosch Mixer is More Efficient: The second major problem with spur gears is that even before they break, they transmit torque poorly. Torque is the ability to exert power at low turning speed — exactly what's required of a good mixer. The Universal's competition starts with a much smaller motor, then spends a big share of it on friction. Which is why many big mixers have surprisingly little oomph.
The Bosch Belt: Universal's transmission uses a belt drive design so strong that fewer than 1% have failed within twenty years. And that's twenty years of heavier work than most mixers ever see, because the Universal's reputation as a workhorse causes it to be purchased by a higher percentage of cooks who place high demands on their equipment. The belt drive of the Bosch Mixer also prevents motor failures from the heat that develops in the cramped gear enclosures of overhead drive machines, a major cause of "mixer meltdown" in other machines.
Kitchen Aid Mixer
If you have a Kitchen Aid, certainly continue to use it and be sure you have the right attachments for the job, but if you are looking into buying a mixer, I would have to say to go for the Bosch.
The price of a top of the line Kitchen Aid is $499.99 (575 Watts) http://www.kitchenaid.com/flash.cmd?/#/product/KL26M8XOB/
The price of the Universal Bosch Plus (which is a step above what I have at 700 W) is $499.98 (800 Watts)http://www.pleasanthillgrain.com/Bosch_Universal_Plus_Mixer_MUM6N10UC.aspx
You decide, and happy mixing.