Hi Mingo,
The total simulation time will be identical according to your settings. However, in many cases it is highly recommendable to use the automatic time stepping scheme: Just imagine that any temporal changes of model properties - like well pumping schemes f.e., induce a change in the matrix system. The solver has to deal with this, so "The greater the change, the smaller the time step" is more or less a general rule for solving. When using constant steps, the solution may not reflect such changes and in some cases, the solution will be less accurate or even not converge properly.
As the name indicates, the automatic time stepping scheme does this on its own. Of course, in some cases further adaption may be needed.
A second good reason for choosing automatic time stepping is the budget: If there is a change of pumping between day 40 and 50 f.e., but your saved .dac file only has step 30 and step 60, you will not see this change in the budgeting process properly.
When thinking about time-steps, you may also refer to this matter as temporal resolution of a model. This is quite as important as a sufficient spatial resolution is.
