DS #1 was 17.5 months when DS #2 was born. 2u2 was very physically demanding - they just both had so many needs still, I frequently had to carry both of them, and I felt like I was pretty much always changing & washing diapers (we did cloth). But in a way their needs were simpler - exhausting, but uncomplicated. This time, my oldest was almost 4 and my middle one was 2.5 when the baby was born. Although they're more self-sufficient in some ways (both are potty trained), in other ways they're more difficult. My mom duties are now more emotionally exhausting than physically exhausting - dealing with tantrums, whining, sibling squabbles, and demands for my attention.
Bottom line, no matter when you decide to introduce a new baby, there will be ups & downs! I'm glad we spaced them how we did. I'm also glad that we're done.