The NIC type is defined in the vCenter Template. Do you expect that the nic type change according with a user selection on the vRA Template?
I guess that the easiest way to do this is having two different templates in vCenter and change the image in vRA according to the user selection.
Or... you can use ABX to change the network adapter after the VM creation, although it seams rather difficult...
I am trying to do the same and I do not see a way to do this in vRA / Aria 8. In vRA 7 we could do this with the custom property VMware.Network.Type. Yes, the primary adapter type is set on the template, but we may want additional nics on a VM with a different adapter type that is not present on the template. Otherwise, we may have template sprawl with all the various combinations of number of NICs and adapter types. For my case we need to optionally provision a second nic as e1000 to support network kernel debugging (as vmxnet3 adapter is still not supported by Microsoft).