Synthetic vs Sable

Natural brushes are a big deal in the brush world, in particular sable brushes.  I have seen art teachers insist on sable brushes in their list of art supplies to obtain for their particular class.  Any new artist will be told to get the best brush they can- a Kolinsky sable- holy grails of the watercolour world (also expensive paper, artist quality paints). 

Pros of Synthetic

• Cheaper
• Easily obtainable
• Not as precious, easily replaceable when worn or lost.
• Cruelty free.
• Some artists think they hold their point and wear better. This probably depends on the way you use the brush.
• Stays the same shape.
• Carries more pigment.
• Good for charging more pigment into areas.
• Softens already painted areas and lifts well.
• Useful for dry brushing.

(Some very popular synthetics are Escoda Versatil, Escoda Prado, Princeton Neptures)

Cons of Sable

• Expensive. People talk about how sables and their fine points last a lifetime, if care is taken.  They don't tell you that "care" involves pretty much not using them.  (Keep them away from water, don't dry them with the ferrule up, don't let water get into the ferrule, don't scrub pan paints with their points).  They DO wear down, they DO wear out.  One daily sketcher I know says she needs a new one every 6 months.  This is one common reason artists hoard art supplies.
• Difficult to obtain. Siberian weasel (the animal Kolinsky sable brushes are made from) are not currently endangered.  But they ARE difficult to get.  Some art stores doubt the ability to offer them at all in the future. They were banned for a brief period in USA 2014. 
• Fact Checking the Kolinsky Sable Brush Ban
• Importing Kolinsky Sable
• Animal cruelty. An animal that would have survived in the wild, freezing winters of Siberia has to die (in an unpleasant manner) just so we can get a few choice hairs from its tail.
• Carries less pigment because it carries more water.

Just to balance the argument a bit:

Pros of Sable

• Soft
• Springy- springs back when you release any pressure on it. More flexible in applications where you are using the brush stroke itself to make shapes, eg making a curved leaf with a single stroke.
• Snaps better. 
• Durable, holds its point longer (assuming you got one that had a good point). 
• Glides across the paper smoothly (skips across the tooth of the paper).
• Carries more water/paint.  There is no need to load the brush as often, saving time when painting. Very easy to use for washes. Good for making juicy puddles of paint.
• Releases paint/ water slower and more uniformly. 
• More moisture control, so you will be able to apply and lift colours and smooth out the transitions more easily. Softens edges well. Eg while painting a very soft transition on a baby.
• More subtle in paint application, giving a more nuanced effect, sometimes greater transparency. 
• Paints a consistent smooth line.  This will improve your painting if you tend to draw with a brush instead of filling in between the lines. 

Cons of Synthetic:

• Derived from petroleum-based products
• With a stiffer synthetic, you push pigment into the tooth
• Disposing of numerous cheap brushes is not environmentally friendly. 
• Points permanently bend easily. Does not keep the point in the long run, but can hold a point to last several paintings.
• Needs frequent replacement.
• Take longer to wash.

In summary, good artists can paint with just about ANYTHING. Think about what you use, and take care of whatever you use, so it lasts.