Microsoft used to say that Domains were a security boundary but now they agree that the forest is really the security boundary. Having multiple domains is mainly used now for isolating traffic to specific regions (ex. East vs West, or Northamerica vs. Europe).
That being said, in many cases the choice to use multiple domains ends up being the wrong choice since it introduces the additional cost of servers and management.
OU's can be configured with delegation to isolate management and security tasks sufficient to meet almost any inter-forest security requirements.
My recommendation is that unless you have a siginificant network connectivitity difference between two separate locations stay with a single domain.
UPDATE: I also forgot to mention that Windows 2008 allows for OU level Password policies, making one of the primary reasons used for separate domains obsolete.