If you run novaclient with --debug, it'll show you the API requests that it's making behind the scenes:

nova --debug boot --image <image_id> --flavor m1.tiny --security-groups sg1,sg2 test

The relevant bits are:

curl -i http://localhost:8774/v1.1/b32b3fad7bfe4b82a353a4374809476e/servers -X POST -H "X-Auth-Project-Id: demo" -H "User-Agent: python-novaclient" -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: ********" -d '{"server": {"name": "test", "imageRef": "1123f214-94ef-4f4c-813c-ffd5e1209aa1", "flavorRef": "1", "max_count": 1, "min_count": 1, "security_groups": [{"name": "sg1"}, {"name": "sg2"}]}}'

Which looks like your second example.

An example with a network:

curl -i http://localhost:8774/v1.1/b32b3fad7bfe4b82a353a4374809476e/servers -X POST -H "X-Auth-Project-Id: demo" -H "User-Agent: python-novaclient" -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: ********" -d '{"server": {"name": "test", "imageRef": "1123f214-94ef-4f4c-813c-ffd5e1209aa1", "flavorRef": "1", "max_count": 1, "min_count": 1, "security_groups": [{"name": "sg1"}, {"name": "sg2"}], "networks": [{"uuid": "07de3a763a3c452c85ffc1008dbe10b8"}] }}'