Well, 8-ball has a lot more traffic, especially at the beginning of the run. Then again, during the beginning stage when you're navigating through 14 or 15 balls and breaking up clusters, you have options and "insurance balls", and the luxury of being able to play position on multiple balls at once and then plan based on which ball you fell best on.
9-ball has fewer clusters, but breaking them up is often much more challenging to accomplish, because you have to play position on one specific ball while shooting your break-out shot, and playing accurate position while bumping balls is exceedingly difficult. Furthermore, if you botch position in 9-ball, there's no possibility of lucking out by falling well on a different ball. You have to shoot the low one, no matter what kind of look you left yourself.
Defense is generally easier in 9-ball, but not always. You only have one ball to hide your opponent from, but then again, that ball is the one you have to hit on the safety shot, meaning most safeties have to involve some speed to separate the OB and CB. This is in contrast to many (most?) 8-ball safeties, where you tap a solid to stay behind it, blocking any shot on the stripes, or maybe shoot a stop-shot, not having to be super careful about where the OB goes because it's not the ball you're hiding them from.
And then there's the break. If you break in 8-ball, sink two solids and a stripe, and get a good spread, your out is likely to be significantly easier than a 9-ball run where you've sunk two balls and gotten a good spread. This is because in that 8-ball run-out, someone with good pattern play probably never has to move the CB much, whereas the low-ball rule in 9-ball means you're probably going back and forth from one end of the table to the other from shot to shot at some point in the run.
On the contrary, if you break weaker and get a little less spread and maybe only make one ball, the 9-ball rack might only have one cluster, whereas the 8-ball rack will have several and the run-out will become next to impossible.
So I think 9-ball break and runs are always a little tough to fairly tough, whereas 8-ball break and runs can be super easy or almost impossible depending on the layout. I think only compiled statistics can really tell the tale as to which one is "easier on average".
-Andrew