I've completed both games!
The flood really annoy me, but just use a bit of rapid fire using the assault rifle on them, and you'll defeat them easily.
Halo II is amazing for multiplayer. My favourite is 'Coagualtion' (Also on Halo 1, known as Blood Gulch,) only in 'Coagulation', the graphics have been enhanced (obviously), there are more hiding places, and a secret chamber underneath each base where there is a Banshee. Its great to have the vehicles on 'Random' so that you have a wide variety of vehicles.
Another good map on Halo II is the 'Waterworks'. Try them out
Halo II is best for mass destruction, with the new vehicles, homing missiles on the rocket launchers, and tougher tanks so you cant be picked off by a few bullets. However, for hand to hand pistol combat, I prefer Halo I. 'Hang em High' is a great map to play hand to hand on Halo I. Its a shame they didnt include that map on Halo II