btcd, and presumably other bitcoin node implementations, seem to have 4 network options:

1. mainnet
2. testnet
3. regtest
4. simnet.

I get that mainnet and testnet both are "public" in that your node will communicate with other nodes and that both have an existing blockchain and genesis block.

However, from what I can tell, both simenet and regtest both create a local only blockchain, and both will create a new genesis block. I this case, what is the difference, and which is better (easier) for developing bitcoin applications?

This is what the documentation has to say:

--regtest             Use the regression test network
--simnet              Use the simulation test network