Is the difference between split view and slide over just that split view splits the screen and slide view allows you to slide the window to either side of the screen?
There’s a few differences, and I may miss some.
1) Slideover is fixed size. Although, as you note, you can move it from side to side you cannot change its size. You can slide it off screen to the right temporarily and summon it back quickly by swiping from the right.
2) Split view is resizable. You can merge it initially on the left or right and then resize 50/50 or 1/3 to 2/3rd on either side. Unlike slideover if you dismiss a split app by resizing it to 0 width it cannot be brought back without repeating the procedure.
With the above in mind I’d say slideover is best for things which you want to refer to often but briefly (maybe a reference sheet or Messages, while youre primarily doing something else) and split is good for bringing in longer running tasks, especially ones where you might want to do a lot of dragging and dropping between them (say, splitting a Word and Excel session)
Don’t forget you can have both 2 apps in split, and one in slideover simultaneously. You can even have a Picture in Picture video at the same time. However, only the newest iPad pros can do all four at one time. If you have one full screen app, one slideover and one PiP they can all be running actively on all iPads. On all but the newest Pros if you have two apps in split and bring in a slideover the split apps grey out temporarily and suspend their work, until the slideover is dismissed.