Instore is better than premixed I think
I've had the opposite experience - pre mixed ones will be done by machine in a factory, so should be fairly consistent (although sometimes consistently wrong!)
The in store stuff relies on a guy measuring the different mix colours by hand and pouring them into a container. If you get a really good, experienced and anal guy on the paint desk then you could get a close match. If you have the part-time teenager with little interest in cars, earning a bit in Halfords to pay for the weekend booze, you may not be so lucky.
Go to VW for paint if you can afford it, or try someone like Paints 4U - as said above spray a little paint in a container and use a toothpick.