There are a lot of things to think about and options to weigh, but I think my most important advice to someone facing the agent decision is: Know Thyself. Know what you want. Do you want an agent to hold your hand or one to just sell your work? Do you want one who edits or one who doesn't even read your material?
There are no right or wrong answers. Different writers want or need different things. And an agent that can be perfect for your best friend may be terrible for you.