bitcoin node: what is the difference between simnet and regtest?

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

  1. main network
  2. testnet
  3. regtest
  4. simnet

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

However, as far as I can tell, both syimenet and regtest create a single local blockchain, and both will create a new block of genesis. In this case, what is the difference and which is better (easier) to develop bitcoin applications?

This is what the documentation has to say:

--regest Use the regression test network
--simnet uses the simulation test network