What is the difference between impact vs regular ratchet/breaker bar sockets? They look similarly shaped and their corresponding tool drives seem to be of the same dimensions. Are the impact stronger? If I have impact in one size, do I need regular for using them on manual ratchets or breaker bars?
To my knowledge the difference is down to the material used
Impact sockets look black because the surface is carbonized (aka drop-forged) in order to harden the surface. The surface-hardening enables impact sockets to absorb sudden torque changes (aka "impact") better.
With high enough impact, a regular socket may warp out of shape because the steel used there is softer.
Cost aside, I cannot think of any drawbacks for using an impact socket with a regular ratchet or breaker bar, just that it is a bit overkill (similar to using a space-grade ballpoint pen to jot down notes on Earth - the ability to roll out ink without relying on gravity is wasted).
Impact sockets are made with a softer steel that can handle impacts better. They are thicker because the steel is in fact softer and easier to bend (but not necessarily break). The reason softer steel can take impacts better is that as you impact it, the entire piece of metal compresses a tiny bit, dispersing the energy of the impact throughout the entire socket.
Chrome sockets are made from a harder steel that is also stronger, but it is more brittle and cannot take impacts well. When you impact harder steel, the energy from the impact doesn't disperse as well, and you end up with all the force of the impact being absorbed by a very small portion of the socket (very likely just the surface of it). This creates more stress even though the total force is the same, and causes a crack to form. Once the crack starts, it propagates and the socket can shatter.
Remember, stress is equal to force divided by cross-sectional area. It describes how much force a certain amount of a material is subjected to. Softer metals allow the force to be shared by the entire piece of metal more easily. More brittle metals can withstand more force if you load them up slowly, but when you impact them they can shatter.
It's also worth mentioning that impact sockets wear out faster than chrome sockets, especially if you use them with a normal ratchet. Because the metal is softer, it wears away and deforms a very tiny bit when you use it, and this adds up over the years. Chrome sockets are still worth keeping around--they will last you a lot longer if you use them properly.
Something which has not been mentioned in the other two posts about impact sockets is the mass. While the material used for impact sockets is more robust and will stand up to a lot more abuse, the real reason there is more material in the socket is for mass. The impact gun throws the mass of the socket ahead for a brief second. The more mass the socket has, the more of an impact it will make against the fastener. This has to do with physics and Newton's second law of motion:
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
In other words: More mass = Greater impact
This bears out in the following impact socket, which is used for taking the harmonic balancer bolt out of the Honda engines:
You'll notice the wall design is about 3x the thickness of most impact sockets. This isn't because it needs to be stronger, but because of the added mass involved in the overall construction. Trying to use a regular impact on the bolt can work, but not all the time. Using this socket on them works every time. Without something like this, you are relegated to using a breaker bar with a long cheater pipe, then blocking the engine so it won't move. If only one person is involved, this is quite a chore. It's all due to the mass of the socket, not because it needs to be stronger or some other reason.
In addition, (from the product description):
Heavy-duty Socket Construction:
...drive impact sockets that feature a rugged, heavy-duty chrome vanadium steel construction with a corrosion-resistant black electroplate finish. These six-point, beveled-nose sockets lock onto drive tang for secure use. And they meet or exceed American National Standards Institute (ANSI) specifications.
Flank Drive Socket Design:
...impact sockets have a flank drive design that maximizes torque while eliminating damage to fastener. A basic socket relies almost entirely on contact with the corners of a fastener. Under high stress, the socket can begin to slip on the fastener and quickly round off its corners. Instead of contacting corners, which have little surface area, flank drive grips the flat sides of fastener, which have a much broader surface area. Large amounts of torque can be safely delivered to the fastener without risk of corner round off. The flank drive design also reduces the stresses on the socket itself for a longer tool life.
Yes this is in fact the correct answer. There are many people unaware of this. Impact sockets are thicker, heavier (for both Mass and resistance to deformation and primarily for safety). In addition, they also save the anvil (Square Drive) on your impact wrenches. Regular sockets will rapidly wear out the anvil and could even break it off completely. Though they do tend to wear out faster under impact use, it's unlikely you would wear them out if using only with hand tools, the real issue is that they are so thick, that they often don't fit or are too bulky in many tight places. Though they are softer, they are still much harder than even grade 9 Bolts, it's all about Safety