Definitely take the time to teach the new players how to play. Assuming you have 3 veterans available to help and coach, I recommend difficulty 4 with an Interesting sector. This gives you a bigger variety of opponents but enough minefields, monsters, and black holes that the players can use as resources.
I also recommend allowing plenty of time for playing, at least three hours. This means people will be hungry so you should have finger foods available. Things like carrots, egg rolls, pizza, sandwiches, or chicken nuggets can be eaten while playing. Things like soup, steak, and spaghetti are much harder to eat while playing.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).