Couple of questions:
1. Are you looking for used or new? A new A4 Cabrio costs a good $10k (USD) more than an Eos, though it is the same chasis and comes with a ragtop vs. a hardtop.
2. Do you need AWD for the winter? Then the A4 would be a better choice with the quattro AWD.
3. Have you considered a 2-seater? In that case, the Mercedes SLK, BMW Z4, and Porsche Boxster are worth a look, though they are a little pricier... then again, a Mazda Miata could be fun, as well as a Pontiac Solstice or Nissan Z 'vert... Good 4-seaters to look into are the Mercedes CLK, BMW 3-series, and the Volvo C70.
4. What is the budget you have for this car?
If you do have it narrowed down to the Eos and A4 cabrio, then the real answer is - what car suits your needs better? The A4 has a little more luxury, and a 'name.' and is available with AWD, while the Eos has the slick hardtop and a cheaper base price. Both are built on the same chassis and essentially the same drivetrain (IIRC, the 2.0T is a bored and stroked 1.8T motor, and the 1.8T motor is a proven powerplant).
For my wife, the Eos was a better fit in various terms including safety, theft prevention, looks, and price.