IMHO it's definitely better to start with normal XP (or TotSC boosted) as ToB XP is what you should have at the END of SoA. Taking a lvl17 mage through Irenicus' dungeon means: Death Spell on the Kobolds, ADhW on the annoying mephits in the Air Pocket Plane,...
You will soon see that it is just to overpowered. Some classes can even get HLAs at the beginning of ToB and GWW is just overkill even for the fearsome Oltyough.
