I found the early years harder physically because of the disrupted sleep, teething, potty training, and the fact the children needed so much of your attention. But nursery was 8am - 6pm and I have cover over all that time if I wanted to use it fully.
However, I find the school years (particularly primary) more difficult logistically. At primary school, they are not old enough to do things by themselves or get to places by themselves. They still need quite a lot of input from parents, in terms of school finishing at 3pm, school holidays, homework, playdates, afterschool activities, outside school activities, holiday activities and the plays (and their costumes!), nativities, assemblies, parent-teacher meetings, sports day etc that Mrs Young described. The admin that comes home in my dcs' schoolbags is unbelievable. You have to be very organised to stay on top of all that.
A professional nanny can do some of that, but they are expensive. I have a live-in aupair that does the schoolrun and afterschool activities and does backup when the children are ill, but the admin and attending plays etc still has to be done by me and dh.
Also, I find there is a lot more socialising between parents at the schoolgate than at nursery. So it is good to do the schoolrun occasionally - I work one day a week from home to allow me to do this - to catch up with the regulars at the schoolgate and get the latest intelligence. Right now, I am looking at 7+ entry for my ds, so it helps to tap into the intelligence network of schools in the area. I am good mates with a few super-engaged SAHM mummys who love to tell me what the best activities, teachers, school uniform sizes are which I lap up in the limited time I have with them.