Id say it's a bit of a mix between the two. It's not quite as hardcore as Post Scriptum but probably a little more hardcore than RO2. My problem with Post Scriptum is beside the guns/weapons it didn't really feel that much like World War 2, Hell Let Loose nails the WW2 vibes... the maps are very convincing, huge open fields, they feel like real locations (because they are) rather than maps designed for a game. This can be frustrating for some as there are huge areas of open space, big fields etc and not much cover... but that's accurate for the battles they are trying to recreate (Foy etc).
Its a bit bare bones at the moment but I had an awful lot of fun in the last beta after not really expecting to, same goes for my mates who all went out and bought it afterwards. It's the surprise game of the year for me.
The dismemberment really adds to the gritty nature of it as well, seeing one grenade turn 4 soldiers into a pile of limbs and blood is quite something.