@EternalCompression has summarized it very well.
It is almost impossible for anyone to tell you what unit to buy, that is why your instructors are being very neutral about it.
Asking what unit to buy is like posting on this board "what car should I buy?" You will get a plethora of answers and reasons but remember that might not be the reasons for you personally.
About the only thing you can do (for the general off the shelf manufactured units) is kind of group the units like:
Back mounted
Chest mounted
Sidemount
The sidewinder style (gets its own category)
I will say that generally speaking (but might not be your case) the units you have listed are a very good starting points you can not go wrong with any of them. A backmounted unit is kind of the SUV of rebreathers. Vey good robust general purpose units. Just strap it on and go. Also very easy to configure for all kinds of diving with 2, 3, 4 on board tanks, and 1, 2, 3, 4, etc. bail outs.
When you start getting into the chest, sidemount, and SW style units they start to be come more specialized units.