Watcher's Keep is accessible both in SoA and in ToB. It appears in both world maps, although you can't travel to Tethyr (I think that's the region in ToB) to Amn and vice versa. You can start the quests in WK in SoA, finish SoA, transit to ToB, and carry on from where you left off at WK, although you still can't return to Amn.
ToB adds a new mage class - Wild mage. Lots of cool weapons AND mundane stuff that makes your inventory management less of a hassle, like the potion case. My characters were lugging lots of bottles, taking up inventory sapce. Now all potions are kept in the case, much better. I also heard that there are ammo belts, etc. But you can only get these in ToB proper.
ToB also adds new abilites for various classes and new spells for priests and mages.
But you should install ToB even if you just started SoA because the exp cap is raised to 8 mil. If you do every quest you can in SoA, you'll hit the original exp cap of 2.95 mil beofre the final battle.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
