Speed - With sewing machines and longarms, speed is recorded with SPM - Stitches Per Minute.
As you can see in the chart below, these longarms are very similar on speed.
Monitor - The monitor gives you control over the machine to select your stitch regulator type (Cruise or Precise) and the stitch length (I like 16 stitches per inch).
The Qnique 19 and 15 PRO both come with touchscreen monitors that are easier to navigate and select your stitches than the Qnique 15R.
The 15R and 21 I'm using in the above video don't have a touchscreen. Instead I use the buttons on the handlebars of my longarm to navigate through the menus. I have heard from a few quilters that find this challenging.
So if you want the easiest way to select your stitches, definitely pick between the Qnique 19 and 15PRO.