Honestly, there are already about 20 different threads discussing this topic, so I would just join in one of the existing active discussions.
Smart bulbs are good, smart switches are good, they have different pros and cons and many people will use some of both in their homes depending on the exact use cases.
The shortest answer to your question is that:
A) smart bulbs allow for color changes (including cool white to warm white), can be used to create lighting zones in areas which have only one switch, and don’t require any wiring.
B) Smart switches are intuitive for guests and young children, won’t come on to full power after power interruption, can be used to control anything that SmartThings controls including Mode changes and the equivalent of scene triggers, and mains powered switches will act as reliable repeaters for other devices of the same protocol. (Smart bulbs will reliably repeat for other smart bulbs, but may not be reliable Repeaters for sensors.)
As far as cost, it depends on the specific situation. If a single bulb is all you need, the bulb will probably be less expensive. If the lighting zone requires three or more bulbs, such as is typical for kitchens and basements, then a Smart switch will usually be less expensive.
Anyway, please feel free to join in any of the existing conversations, there are many of them. 
(The following Thread title is a clickable link.)