Besides some feature difference, also consider the following.
A controller simplifies VLANs at the edge. You do not have to worry about deploying VLANs.
A controller will manage both your wired and wireless Aruba network. All clients, wired and wireless, have the same policy.
A controller provide remote functionality such as VPN and Remote APs to extend your network.
Aruba Instant will scale, now beyond 16APs, but are limited to a L2 subnet. Using basic network architecture rules, A class C subnet is 256 users. It's best to keep a subnet to that limit. For a Class B flat network, while IAP and Controllers will both manage much more than 256 users, you should be careful. Large L2 subnets create a lot of chatter and the network can and will have issues.
As you have noticed, AI will continue to expand it's feature set and is closing the gap with the controller, such as L3 roaming between virtual controllers (not relevant within a group of 16 IAPs), and spectrum analysis, or even logs for troubleshooting. At the end of the day, it depends on what you are looking for.
Hope that helps!