LDK is great if you know a great deal about lightning and are building an application that needs custom lightning behavior.

LDK Node is a good drop in solution if you're building a mobile app where users have a non-custodial wallet that runs on the device. Good if you are fine with their defaults. IMO, it's early, but it should be a better solution than dropping LND into a mobile app, which has historically been a huge pain point and some are moving off of it.

LND is good if you're just a user of lightning and you want a node, or if you're building something that runs a server and you want to communicate with your own always online lightning node from that server.