I'd definitely go for a child a similar age ... my two boys are 22 months apart, and having a playmate round for one of them has, at every stage, been easier than having the two of them together. With two a similar age you can do activities together, play games together much, much easier. If you want to get paints out or playdough for your son, it's no less effort for two, but two get the benefit ...
A baby will spend some time sleeping, but they take AGES to feed, and then you have all the nappies to change ...
Also, your son will soon want to start playing with smaller toys - 'proper' Lego, hama beads, Playmobil, and so on. If you mind a baby, then you'll have to put all this stuff away when the baby's in the house.