I need my users to be in different security groups and the ability to delegate control. What forest model can I/should I use? And why?
There isn't much detail in the question that would require any special Active Directory Architecture. If all you want is groups and delegation you can simply drop a domain controller in and be alright.
That is a single forest topology with a single domain. It is simple, effective, and takes little to no expertise initially.
Typically if you are asking the question which should I use, you are at a point where you should start out with a simple topology until you have the requisite understanding and experience.