Very different games to me tbh. Squad pve with gun focused combat, but thats where the similarities end. Borderland is crunchy with tons of guns stats and special movesin a RPG open world with a set narrative to go through.

Helldivers has a way smaller arsenal and a tighter mission based combat loop. They have more of a meta narrative that you affect vs being the main protags. The tighter gameplay loop is probably the biggest reason I liked it but haven't play borderlands in forever. Like its very much built for dropping in blowing up your friend and maybe some enemies too, try to do objectives against mounting odds, pass or fail the mission, continue. The fact that weapon progress almost always makes friendly fire a even bigger threat makes it almost a trade off and not a clear path to success.