Appleseed recommends the USGI for a few reasons.
It's inexpensive, lightweight, easy to set up, easy to change for a different shooter, and infinitely adjustable with minimal effort.
For most folks, the change from Loop to Hasty is a simple matter of taking out the loop and hooking up the J-Hook to the rear swivel. Now, if you have biceps like Lee Haney, that theory won't really work lol but for most people it will get you close enough. Then of course to switch back you just disconnect and make your loop. And you can quickly and easily adjust how snug the loop is around your arm.
I tried the RLS. I found the keeper difficult to tighten with one hand while holding a rifle and keeping it safe. It just doesn't want to slide on the nylon, even after spending a movie or two working it to burnish the nylon. And it's not a good position for leverage. I had to have someone help me every time I wanted to use it in a loop configuration. Given that we use the loop for 3 out of 4 AQT stages, that got old FAST. It wouldn't adjust tight enough for me to feel truly "slung up," either. And dude, the setup instructions are HORRIBLE.
Since you've already purchased it, obviously you'll want to run what you have, and the instructors will do their all to help you make it work. But even without Appleseed's official position regarding slings, I'd still recommend the USGI every time.