D100 Dungeon
Positives
• Quick to learn
• Random tables are huge and varied
• Tons of dungeon rooms
• Map system is strict and straightforward and feels like exploration
• One character to manage
• Huge amount of quests/scenarios
• Good replayability
Negatives
• It is possible to be stuck with a bad weapon
• Map system can generate a bad layout
• Random tables do not change between quests/scenarios (with the exception of the monster table which has a sliding window)

Four Against Darkness
Positives
• Expansions add to the game
• Manual is very polished
• Your characters begin with guaranteed usable items
• Can split the party of four characters for co-op play
Negatives
• Map system makes it feel like you're designing a dungeon vs exploring it
• Included random tables are short
• Four characters to manage
• Low replayability of the base game
• Expansions are needed if you want to continue with your party

I actually really enjoyed both games but I'd give the edge to D100 Dungeon.