K2 replaces regular com_content. I use it because it's com_content with more features. Now Joomla core has intro & full image, a feature not available with K2. Now I always have to decided if that intro & full image worth the project or use K2. I only seem to use K2 for blog/magazine style websites. the default com_content is getting more appealing.

The main difference i see in image handeling is that with k2 you can select a different size image for the category and item view. with the j articles you must use images that are sized correctly before uploading. Both ways have advantages and disadvantages