One uses the power of your CPU (the old way) to preview your document, the other uses the GPU (the new way, which uses the power of your graphics card to preview the document, while allowing your CPU to continue to do its thing at the same time).
Newer computers will almost always run faster using GPU preview, while most computers 3 or 4 years old may find that CPU works better (though the type of graphics card can make a big difference.
Zooming and scrolling, screen redrawing and similar functions are what are affected the most here, so it'll be easy to see which works best for you.