My three are all 20mths apart and I found it harder getting used to just having the one child. Erin just fitted in so well and three is a breeze . Seriously, having two seems easier because like mbear said, you are more relaxed plus you are already in somewhat of a routine by the time #2 arrives.

It is totally personal though, some will be run ragged with two and others will cope with 10 kids. I haven't had only one child for long enough to notice that much of a difference, but if I had a second child when the first was several years old, then that would be different.