I actually love Velocicoaster (I'm a bit of a coaster junkie) but it doesn't pretend too much. It's fast and has that awesome section as it cuts in, through, and around the rocklike concrete and waterfall. The second launch is great and, as a coaster, it hits on all cylinders. Due to the more thrilling nature and overall better ride experience (including the queue which was themed pretty well), I would wait 45 minutes to ride Velocicoaster whereas I don't really need to ride Slink Dog again (unless the line is 20 minutes or less). I just don't understand the wait times for Slink Dog aside from the fact that it's one of two coasters at Studios and not thrilling which makes it approved for the younger set. Of course, I love Spaceship Earth, Pirates, Haunted Mansion, Big Thunder, Everest, and most rides at Disney World. UNI/IOA is a one day trip for us but WDW is multiple days and not just for the rides.
However, I was comparing Slinky and Test Track. As you stated perfectly, Test Track needs to be maintained better and could use some onboard audio with the car but I still prefer it over Slinky. It's also difficult because they are totally different rides for a comparison. A better comparison would be maybe Dinosaur vs Test Track or Slinky vs 7 Dwarves Mine Train.