Beginners should always look for the smallest possible turn radius. A good narrow waisted ski should be able to rip an 11-13m turn with ease. Learning how to ski with a thick ski that has a poor turn radius usually means that the ski will be lacking in other characteristics that will be crucial for developing good skills as a beginner.
It is possible to be a great skier on fat powder boards, but too many beginners start there without ever developing a rounded set of skills to properly control their skis. Skis with a smaller turn radius tend to be more agile and easier to control once you master the various aspects of managing your ski successfully. If you get into a hard pack or ice a narrow-waisted ski, it will be a lot easier to bit in and stay safe as well.