1. There are - as you've said - many varieties of Will III silver coins, and rarity will determine the relative prices
2. They are just guides - as you've said - so need to be taken with a pinch of salt, and two eyes on how the prices were compiled
3. Buy one guide and stick to it. If your interests are George III onwards, then CCGB (link in the banner ad above) is both cheap and comprehensive. If your interests are wider British, then Spink's annual catalogue is "the bible". If your interests are global, then Krause, though bear in mind Krause is not comprehensive for varieties, and their prices don't reflect local values but more what a foreigner might pay for coins of a particular country.