You are in a fantastic segment of the car market.
I recently bought a new car (Infiniti G35x), and test drove 2 cars on your list before the purchase- the Acura and the Audi.
They let me take the Acura home overnight to test, which I HIGHLY recommend before making your final decision. Technically, they are renting you the car overnight, so you will need to leave them insurance information and some form of identification. No cost, though. Then, you are free to get friend/family opinions, take them for a ride, and you will definitely have a better idea if you like the car or not than you would if you drove around for 5 minutes with a salesman yammering into your ear.
The Acura is a fabulous car. It's like driving around in a big, plush, leather armchair. Extremely comfortable. Very roomy interior, front and back. Has a huge navigation screen with very attractive graphics, and simple to use, as well. Has every gadget and gizmo available on the car market, from retractable window shades to all-wheel drive, to front/rear seat warmers. OnStar roadside assistance feature. It is considered to be the state of the art, technically speaking, if thats your thing. Strong, quiet V6 engine. (The Audi A6 comes with either a V6 or a V8 engine). Appearances are subjective, of course. I think the car has a sporty, luxurious look to it...maybe a notch below that of the Audi. The car comes one way: with all options available, at just under $50k.
The Audi has, in my experience, the best looking/feeling interior on the market. The layout is sharp, modern and absolutely beautiful. The materials have a luxurious and expensive feel somewhat missing in the Acura. Exterior looks more stylish and sophisticated than the Acura. I drove the V6 version, and found it a bit sluggish. This is a big, heavy car that needs V8 power, which you can get for $10k more. Super comfortable. All wheel drive. It's the more elegant car of the 2, and it will most likely cost more when spec'd out to your liking.
I haven't driven the Lexus, but it has a superb reputation as a luxury car. A little less sporty than the others, more upright and conservative in spirit.
Good luck in your decision, and try try try to get an overnight test drive in whatever your choice of car.