Wifi or Ethernet for my iMac?

You may be a little less on the safe side with WiFi, but you can setup a good password protection.

If you are even on a fast internet connection you will not see any decrease in speed if you connect with WiFi instead of Ethernet. That's why usualle Internet is slower than WiFi connection provided by Airport Extreme. You may have problems if there are many WiFi connection but you can test it.

Ethernet will be for sure much faster if you measure the connection speed locally, that is between 2 or more devices connected locally.

No as long as the wi-fi connection is doing the job just enjoy not having another cable attached! If you were transferring large files between computers in your home Ethernet would be faster however for music and video wi-fi is just fine. One thing you may want to do though is in

System Preferences - Network - Click the upside down triangle in the lower left and choose Set Service Order... and move the wi-fi to the top. Here is a graphic showing:

A wired connection will always be faster and more reliable than wireless.

Further, you can reduce your exposure to RF radiation.

rkaufmann, why do I want to move the Wifi to the top of the list?

The service order is the priorty order, so because you're using Wi-Fi it will give the system a miniscule (probably not even perceptable) speed bump because that is what it will look for first.