Stanislaw Lem was Polish so it depends if, by Soviet SF you mean something like 'Communist Bloc' SF.
I've recently read some of the Strugatski brothers which I really rate, Hard to be a God and Roadside Picnic (which was filmed as Stalker, which then I believe help inspire the video game.)
It's definitey literary SF, so you have been warned, but usually they attempted "darker themes and social satire". To be honest, I feel they were 'in step' with other SF movements in the West at the time (Ballard was publishing at exactly the same time for example, for which I can see parallels), so really I wouldn't have said there was a difference there. Reading some of the brother's memories of trying to get books published, I would guess there would have been a lot less 'pulp' SF around in the Soviet Union, but that's a pure guess.