I have both these sets... and both are somewhat underwhelming to build. The skeleton one, in particular, feels out of scale with the castle in the same theme, but this is a charge that can be levied against both. Fig-wise, the Royal Knights have the slight edge, since (in part due to the Royal Knights) I've never been able to consider skeletons as "real" minifigs--more like the Castle version of battledroids. On the other hand, skeleton horse are AWESOME while plain white old-fashioned horses were as vanilla as they came--especially without any saddles.
The skeletons have the edge on the playability side. This is a bad thing where the design of the carriage is concerned, because it means a lot of less-than-useful parts (all those spikes and horns!), as well as a distinctly unlovely image. The skeleton carriage is, on that note, NOT a display set. On the other hand, it comes with an opposing knight, which provides instant conflict, and it has always seemed plausible to me that the princess seen kidnapped in the skull mountain set was probably transported there in this carriage. Meanwhile, the Royal Knights have... a treasure chest? By 1995, those were hardly new.
So it's a bit of a toss-up. Nostalgia wins flat out for me, but taking things more objectively, these are each somewhat mediocre.