Splitters make more sense to me since different things should be categorized differently.
However, I believe a major problem in modern computing is when the splitter becomes an "abstraction-splitter."
For example, take the mouse. The mouse is used to control the mouse cursor, and that's very easy to understand. But we also have other devices that can control the mouse cursor, such as the stylus and touchscreen devices.
A lumper would just say that all these types of devices are "mouses" since they behave the same way mouses do, while a splitter would come up with some stupid term like "pointing devices" and then further split it into "precise pointing devices" and "coarse pointing devices" ensuring that nobody has absolutely no idea what they are talking about.
As modern hardware and software keeps getting built on piles and piles of abstractions, I feel this problem keeps getting worse.