I think with both racks you will achieve more of a vertical gradiant that you anticipate. I assume with the CC rack you have the option of only weaving the heat rope through the tiers you need so that may be a better option.
Also, I'd rather trust HDPE to E. mandarina and Lamprophis than expanded PVC. The HDPE will conduct heat less as well possibly adding to its attributes for Mandarins.
But the security of the rack is greatly dependent on the gap built between the box and the shelf - this may vary between the two companies. Assuming it is equal, I'd prefer the HDPE for its rigidity.
With the Boaphile racks you have the option of boxes that slide in width-wise or length-wise. With the length-wise you'll obviously allow more opportunities to the Mandarins to get away from the heat. I'm not sure if the CC offer this option.
Sorry to hear your friend is a flake. Building your own sounds like the perfect solution for the collection you have.
Don't rule out the option of paying a bit extra and having all the wood cut for you and then assembling it yourself. This will still be a cheaper (although heavier) solution for your needs.
If you do this I'd go with a cabinet specialty shop as the cuts will be a lot more accurate and make assembly easier.
Maybe you can get your friend to snap out of it long enough to at least get you through the initial assembly stages.
And since you won't be restricted by shipping you can build the rack to be 7 tiers tall and make better use of the material. In case you've not heard me post on this before, seven 28 qt. rubbermaids will fit in a 4' tall rack and most materials come in 4' increments. However, rack companies don't build 7 tier racks due to shipping constraints.