I was looking over the different weapon styles, and it seems to me that two slots in single weapon style helps your armor class more than a shield would. Is this worth spending slots on, or should I stick to a shield?

Single weapon style only helps you until you start getting magical
shields. If you plan on using a bow, then throw some slots into
your single weapon style. Then when you switch from ranged to
melee, your ac drops to help you.

If you must use a sling, then use the shield style.

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If things got any better, I'd be twins.
Keeper of Lost Cause

There are some shields in the game that provide a MAJOR improvement to AC, much more than Single Weapon Style would. I would say Single Weapon Style is best used by classes that have limited access to shields, such as Thieves and Bards, or if you are going for a them, such as Shadowkeeping an Elf Fighter-Mage so that he can attain Grandmastery in one weapon ala the Complete Elves Handbook.

Other than that, there is a lot of AC improving equipment in the game, including weapons, which allows any weapon style to be viable, even Two-Handed style (i.e. one-weapon in two hands, no shield). The styles I feel, are more of a role-playing hook than anything else, as their bonuses (other than two-weapon which grants better Thac0 while giving an extra attack, quite useful) are relatively mediocre.

Its's really all up to your style of play. Remember though, that most classes/kits cannot get more than 1 star in Two-Weapon Style, and at later levels may mean you miss twice instead of hitting once...