My twins are 13 and I have 2 older boys - 16 and 19. Twins run in my family (all my siblings are twins, I am not), so I conceived them naturally. I have absolutely LOVED having twins. It was seriously busy in my household when they were born - and I had no help from relatives or friends, but DH was great. Mine are all boys so they are best friends and always have each other to hang out with. Even at 13, they are rarely in different rooms in the house.
Diapers are expensive, but the expense does get worse as they get older. There are some things that we just couldn't do because it would have been double - like club sports. And I'm already scared to death about having 2 go through college together all 4 years. But I try not to think about that.
Anyway, I wish you luck with your decision, but twins are amazing and if it were my decision, I'd go for the twins.