Even if you're still playing through SoA it's well worth getting as it will increase the maximum amount of EXP points you are able to attain. Plus you will be able to visit the massive dungeon The Watchers Keep straight away if you desire.
The new spells available include some truly awesome 10th level spells for mages (such as Comet) and quest level spells for priests (such as Storm of Vengeance). There are also powerful foes to test your mettle against, including several Dragons. In some cases it's not the quality of your foes that's staggering, but the quantity (you have to fight a small army at some points in the game
). The only new kit is the Wild Mage, who uses wild magic to create some very interesting (and often hilarious) effects.
ToB also concludes the Bhaal-Spawn saga, and any romances you might be indulging in. Overall - a very worthy purchase!
To answer your other question: Fatigue is based on constitution. Aerie and Jan (I'm assuming) both have fairly low constitution, so they will get tired quicker then the rest of your party.
------------------
"Our deepest fear is not that we are inadequate.
Our deepest fear is that we are powerful beyond measure."