Such a tough one, for sure, Teta! Prior to my 3dt there was no way I could imagine doing a single embryo transfer. I'm 31, no other kids. Only after the first ultrasound showed twins did my doctor say that a person in my situation had a 40% chance of getting pregnant with twins by putting two back. Ask your doctor what he thinks of your situation.
It's tough, though. IVF has at most a 50% success rate. But of that 50%, you have a 40% chance of twins if it does work according to my doctor.... If you KNEW FOR SURE it would work, it's a no brainer to do a SET.
But ...yeah... the thought of it not working at all, especially when you've paid out of pocket.... Like sunshine said, it's an entirely personal decision. You and your OH have to make it together with the guidance of your doctor. And be mentally prepared for whatever outcome as best you can.
Good luck, hun!!