The real thing is planning your inning, all the way through! Eight ball or nine, it is easy to get lazy and not plan all the way through an inning. Are you planning to pocket the money ball this inning? Does it look like you will have to play a safety? Either way, have it planned before you hit the first ball. I see a lot of people not get past the three ball concept thinking that they only need to plan three balls ahead when that cluster is waiting to wreck their plans. You don't need to deal with the problems first as some say, however, you do need a plan to deal with the problems. Silly to try to force the cue ball to break up a cluster shooting the one or two when shooting the five ball is going to give a natural path to break out the cluster.
If you aren't working with a complete plan, pull balls off the table until you are. Cumbersome but as a start you work every shot back from the money ball then forward through that same path. If you can't do this with nine or eight ball, take some balls off the table after the break when practicing. You need to see a pattern, or two or three, before hitting the first ball. Unless you have a firm idea of what you want to do, you are jumping the gun hitting the first ball.
To me, neither eight or nine is harder. I like eight ball a little better because you can be more creative but I might say nine or ten is harder. I agree with those saying shoot ten ball as long as you still see entire patterns. I think it has reached the point that top level pro's should be playing twelve or fifteen ball instead of nine or ten in competition. Orcollo's ten or twelve rack run was awesome put he was mostly shooting the same pattern over and over. Making two, sometimes three balls on the break he was usually looking at a seven ball pattern. The balls were no doubt clean and freshly polished which makes them open up nicely. It was almost a given that he was out after every break. That is a problem playing any game in my opinion.
When one game starts seeming less challenging, play another. To really frustrate yourself, play one pocket in practice, strong hand against your weak hand. Start off giving your weak hand a spot.
Hu