A very basic comparison:
http://www.newcars.com/reviews/mercedesbenz-slkclass-vs-audi-tt-convertible.html
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Comfort
The Mercedes-Benz SLK-Class will be a favourite with taller drivers due to its greater front head- and legroom than the Audi TT.
Convenience
You'll be making about the same number of stops at the gas station with the Mercedes-Benz SLK-Class as you will with the Audi TT, as they get the same number of miles per tank of gas.
Cost
There are a lot of factors to consider when calculating the overall cost of a vehicle. The main consideration is Manufacturer's Suggested Retail Price (MSRP). The Mercedes-Benz SLK-Class is somewhat more expensive than the Audi TT. Another consideration is the Destination Charge, which is a standard charge for transporting the vehicle to the dealer from where it's built. The Mercedes-Benz SLK-Class and the Audi TT have similar Destination Charges.
Dimensions
Since the Mercedes-Benz SLK-Class is much slimmer than the Audi TT, it'll be much easier to find a wide enough space in a crowded parking lot. When looking at overall length, the Mercedes-Benz SLK-Class takes up about the same amount of space in your garage as the Audi TT.
Drivetrain
With its lower torque, the Mercedes-Benz SLK-Class doesn't do as good a job transmitting power to its wheels as the Audi TT.
Handling
The Mercedes-Benz SLK-Class has a smaller turning radius than the Audi TT, allowing you to more easily manoeuvre in and out of tight spots.
Performance
When it comes to horsepower, the Mercedes-Benz SLK-Class is somewhat more powerful than the Audi TT.
Utility
In terms of seating you'll be able to fit the same number of people in both the Mercedes-Benz SLK-Class and the Audi TT. The Mercedes-Benz SLK-Class has a roomier cargo area for holding your bags or other items than the Audi TT.
Warranty
The Mercedes-Benz SLK-Class comes with essentially the same basic warranty as the Audi TT.
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The Audi has Quattro (all wheel drive), so it'll probably have better grip and better cornering, but the SLK is a bit more powerful.
Take both for a test drive and see which one you like best.