Not much difference compared to the first one which is a good thing.

They did some changes. Reloading no longer requires you to aim. Items no longer drop when enemies die unless shot at or stomped on.

If you manage to get to disc 2 you'll get a free checkpoint at chapter 7 since the game won't ask you to insert disc 1 to reload your last save if you die but it must be continuous play of course. I already completed Hardcore, it's not really that hard considering the difficulty is actually survivalist enemies with zealot loots if you've done zealot.

Very fun to use but it doesn't have a laser sight. Especially useful later on when there are tons of necros storming you.

Just get the second one, there's a 'last time on dead space' thing that will tell you what happened in the first game. This is a fantastic game that shouldn't be missed.

Severed is about 30 minutes to 1 hour of gameplay. Pretty much the same like the main game. You'll get Gabe's Seeker Rifle and his rig for Isaac's story but they are pretty much useless imo. 90% of the environments and enemies are taken from the main game. Kind of regretted buying it since it doesn't do anything to the main storyline whatsoever.