I have a friend who is on the hiring panel with a federal agency as a 1811. We had this exact conversation. Now his agency is not one of the big ones (FBI, USSS, DEA, Customs), but they do hire 1881's. He stated that a certificate will put you "...slightly ahead of a canidate with a bachelors, but the masters takes the cake." From what I got, the certificates are are great to pursue, but actual credits and a degree are what is needed.
Thus...I am back in school this semester startingto pursue my masters. I know it seems hard with all the shift work and crazy schedules, but it can be done. Look at online classes at ACCREDITATED universisties. Most schools now offer them. Even big name ones like Syracuse University does. And from what research I havedone, nobody cares if the course was online or in-person, just if it was an accreditated university. And if that dosesn't work, talk to your boss. Even here is the huge NYPD, where you are just a number, they will accomodate you for school. Some guys have classes on a specific day and if they are supposed to wrok that day, they work a specified tour. Such as day tour personnel who take day time classes work a 1600x2400 that day, for the entire semester. If the uncompromising NYPD will work with you, I am sure most other departments will.
Certificte good for personal knowledge....but don't expectto get much mileage out of a job interview with it. They will ask "If you were able to do 5 classes, couldn't you have done 15 more and got the masters?"
Good Luck!
-In God we trust. All others, put your hands on the car and don't move.