Hi linzykins,
The weakest link in a mattress is generally in the softer comfort layers not in the deeper support core so an innerspring isn’t normally a weak link in a mattress. There is more about latex vs innerspring support cores in post #13 here.
They would be very closely comparable in terms of durability and performance but Spindle uses 100% natural continuous pour Dunlop which is a more costly material vs the the 85% natural and 15% synthetic blend used in the Morgongava so it would depend on whether the natural rubber content was important to you. They also use individual 3" layers of latex which can be rearranged or replaced so as you mentioned you would have more options both before and after a purchase to customize the mattress or to fine tune the mattress as your child’s needs and preferences change over time and their cover also uses higher quality and more costly materials as well. Of course they also use more latex which adds to the cost as well.
It may also be worth considering the Spindle 7" “B stock” mattress here.
Spindle does include all the specifics of their mattress construction and design that most people would want to know on their page here (you can just click the different tabs for more specifics). If you have any questions that aren’t included on these pages then they are very knowledgeable and can certainly go into as much additional detail as you wish if you give them a call.
You may have read this already but post #2 here and the topics it links to also has much more information about mattresses and children and “safe” materials including a link to some general guidelines for children in post #2 here. It includes a number of links to the better forum posts and topics about mattress and children as well which also include more information about many good options for children which may also be worth considering in a wide range of prices.
Outside of any other options that may be available locally … some of the better online latex and latex hybrid options I’m aware of that are lower budget ranges listed in posts #3 and #4 here although they don’t include mattresses that would generally be suitable for children only such as the Spindle “B Stock” mattress or the Arizona Premium children’s mattress here.
Phoenix