For microlimit zoom/rush tables i can tell that GGPoker is much tougher than Pokerstars where pokerstars is much tougher than partypoker and 888.
I think on GG there are a lot of (semi)professional micro grinders from the eastern states (russia etc.) and asia.
Softwarewise I'd say Pokerstars and GGPoker both did a very good job, but depending on your design or customization needs you might love the one and hate the other.