Which is better ready mix screed or mix on site screed?

ready mixed gives you more time to lay it ..it has a retarder built in ..but when i started i used to mix by hand ...in a mixer it goes bally ..i used to mix 7 tonnes a day 

It depends upon the brand of the sacked mix.  They are not the same.   What varies is the amount of cement and the amount of aggregate/sand proportions.  The variations affect the strength of the concrete, with 3000 psi concrete being the standard strength.  You can look up and find the recipe for this concrete.   So you have various brands of bagged concrete mix.  One brand Sac-crete (?) has little cement and is used for setting posts in holes.  But, there are other brands that will make real concrete.  Now, best concrete comes from a batch plant where they can control everything and have the proper ingredients.  Like river rock flint gravel, quartz sand, quality cement, and precise water-cement ratios and equipment big enough to handle it all.  Some guy mixing concrete in a mixer on site using a shovel just isn't precise enough to get the same quality as a batch plant.  For one thing, he will be using way too much water.  So, if you are setting fence posts, go to the lumber yard and buy the sacks of the high grade concrete mix.  It will make real concrete for those fence posts.  Also, if your post is wood, raise that good concrete a couple inches above the ground to keep the wood post dry.  Fences fail by the post rotting off at the ground line.  Your fence will last twice as long.  Buy some aluminum flashing  and make a round form and set it in the top of the fence post hole.  Easy to do.

On site is just as good if the right people mix it ready mix is a  computer regulated mix and water to give the correct consitency through out , i have laid floors and ready mix is easier to lay 

Much better guarantee of quality for ready mix, for which you pay a premium obviously.