Both are used to simulate big circuits with more than 1000 elements. They divide the circuit matriz into small ones to achieve high speed. The circuit could use partition, ie different precision in different parts of the total circuit. Hspice uses only one big matriz to solve the circuit, so that it takes more time to simulate large circuits.
As bastos saib both simulators referred before i.e. hsim, nanosim and ultrasim are top level simulators ususly used for circuits with more than 10000 elements this to take a real advantage of is usage.
The big difference between them is:
Hsim ---> The most fast and accurate for digital and analog designs.
Nanosim ---> More fast than Ultrasim and in certain situations can be more acurate
than nanosim.
Ultrasim ---> Is the most slow and the worst interms of accuracy for analog design
meanwhile for digital desgin where the accuracy for now is not so much
important is also very good