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.

LDK still really early stage and maybe offer no-standard functionalities, like CLN. If you're starting, go for LND, there's plenty of documentation, integrations and tutorials

Curious to try LDK but have not yet.

In my experience, LND good for simple standard nodes where as a personal user, or possibly for small family group, want non-custodial lightning for personal use with a very small number of channels; be extremely wary of using it if you want to build a routing node. So much pain.