>>I had wondered when looking at new enclosure options why some companies used one type of plastic vs another
I think to find out why a company uses one type of plastic vs another, you'd probably want to ask the company directly. My guess would be #1) Cost, and #2) Ease of construction. Neither of those mean anything to your animals.
The following are the important (to me), differentiating factors that led me to purchase a cage made from HDPE (I chose white HDPE over black HDPE):
Translucent - which allows me to actually see my animals without installing a light in the cage or using a flashlight.
NON-porous (unique to HDPE and not applicable to black HDPE)
Wipes clean with NO effort - nothing sticks to it at all, even glue (or dried urates). It's like cleaning a glass surface - no scrubbing required.
Doesn't "off gas" which means your herps aren't breathing in small amounts of toxic fumes day after day for their entire lives
Because nothing sticks to it, one piece must be bonded to the next so the cage is WELDED, not "sealed" or "caulked" which actually provides better sealing.
It's a stronger, more durable plastic
That's pretty much it, I believe.
Here's me next to a stack of 5 cages on a 6" pedestal. I just recently ordered 5 more and am waiting for them to be delivered in the next few weeks.

I will be replacing that rack on the left very soon. I'll be replacing it because it's not HDPE and because of that, it off-gasses (visible over time when looking at the side of the tub that is in the back of the rack - there's a thin film of black something-or-other [and it isn't dust] that needs to be wiped off every so often) and it also sags which pinches the tubs and makes it difficult to remove them and push them back in with one hand. I've had that rack for YEARS and it's always sagged (from day one) and it STILL off-gasses.
HDPE will sag too, but the method of construction used from my supplier keeps it from happening.
Hope that helps,
jb
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What's written above is purely my opinion. In fact, MOST of what you read on the internet is someone's opinion. Don't take it too seriously 
Jonathan Brady
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