I think the benefit of an attorney depends on your specific situation. You can prepare the forms properly without an attorney. I prepared the AOS package myself and only hired an attorney after there was absolutely no progress in my case several months after filing. My lawyer wrote several letters, first to the local USCIS office and then higher up the chain of command, until I got an interview scheduled. If your number is high, it might be useful to have a lawyer from the beginning. However, if your number is low and you have a lot of time until Sep 30, you can file by yourself and hire a lawyer only if things don't go smoothly.
Good luck to all.