Simulation with uncertainty

Dear NMusers, I would like to get your thoughts on some common used techniques for simulation with uncertainty. If one is interested in simulating the expected mean response, there are two methods that are can be employed: (1) Use the variance-covariance matrix (2) Use of bootstrap results. What assumptions are we making when using each of the methods? What are the respective prose and cons? Do you have any preference in terms of when to use method 1 over 2 or vice versa? Thanks and kind regards //Dinko