BG2 Question - Watcher's Keep

Hi all,

I'm currently playing through BG2 for the first time (after finishing BGEE I didn't want to wait for BGEE2). Now, I'm in chapter 4 and my characters are already almost maxed outin XP. I was wondering, when is the best time to tackle Watcher's Keep - in BG2 or in TOB? Is the difficulty of the dungeon ramped up in TOB? For which game is the loot better? Thank you.