Very big difference. The supersoldiers in 40k are 8 feet tall, while the Spartans are just a regular 6 feet.
False. Having heavy bolters mounted on motorcycles and speeders is way cooler than building walls or doing Ultramarine stuff.
Warhammer 40k Lore was always much better anyway. Also, as a Single Player only and having played the master chief collection I'd say Halo is overrated. I think most of the greatness was from multiplayer.