Before I say anything, let me preface this with a little note: I use the Complete mods for STALKER and set it up to zombify any NPC Stalkers outside during an emission, so I probably see more of them than most.
That said, STALKER zombies tend to rate slightly below the scenery in the threat pyramid. At worst they're a slight inconvenience anywhere past very early in the game, at best you can pop them and grab their ammo. They give the Zone a bit more life, you'll see patrolling stalkers in Yanov and Zaton getting into occasional gunfights with them, and they make it so you need to think a bit more about what you're doing even if they're no real hazard.
Metro? Metro's totally different. More mutants, nastier environments, more Weird Shit, and very little real civilization (as opposed to STALKER, where you've constantly got people and commerce going in and out of the Zone). The Metro is maybe two dozen habitable stations, and as far as the games and original books go, may be home to the last humans on earth. Plus it's all tunnels, so you don't have the option of avoiding whatever freaks are around, unless you're crazy enough to go to the surface, which is usually worse. In STALKER, if you spot a Pseudogiant stomping around a couple hundred meters off, you can go around it or wait for it to leave. With Metro, you don't usually even see mutants before they're on you, and if you need to (for example) get to Polis during the nosalis migration? Good luck buddy, you're either going through a shitload of ugly-ass mole mutants, or hauling ass through Moscow hiding from Demons.