In term of system ressources, i think Pendor is more demanding than Light&Darkness, but in the same time the ressources devoured by the engine are very linked to the battle size used, even if you have a "not that good" system, with lowering the battle size you can have the mod very playable.
Now in term of content and gameplay, L&D is very different than Pendor, while in Pendor the story and the majority of the events are only going through the dialogues you read from the various people in the taverns (with the new order quests, there's an actual event that is really happening at some point, not only in text) and the rest of the game is not that much different from what you have in Native (you're merc, then vassal, then try to make either your king conquer the map or you're going to rebel and try to conquer it yourself).
In Light&Darkness the story from the mod is actually happening to your character through many quests, and events happening, you don't only read about those, they actually happen in gameplay and most of the time your character is going in.
Even if you have played a lot of mods in Warband, i really suggest to get Light&Darkness, it's truly unique in the Warband mod roster.