All the above mentioned brands of paints are excellent. You can use any of them paint awesome minis.
The problem arises when you need to change brands, for whatever reason, and you are not sure which one to choose. It took me a while to change from my set of Derivan Mini Paints (an Australian made paint range for miniatures), to my now Vallejo range. From my research there is not really an EXACT match between brands of paints, but instead there is very close comparison. So if you were half way through painting an army, then I would continue using your original paint range until that army is finished.
If you are ready to consider other brands of paints to what you are using now, then have a look at these links for some opinions on various paints.
Youtube Videos on the topic of Miniature Piants.
Miniature Painting Fundamentals 2 : PAINTS
A History of Games Workshop Paints - talks about Coat D Arms
General Review - Reaper and Vallejo Paints - Part 1
General Review - GW and P3 Paints - Part 1
General Review - GW and P3 Paints - Part 2
Citadel, Vallejo, or P3? Which Paint do i choose?
Privateer Press formula P3 paint review
Games Workshop paint review
Ghost's Paints [GW Paints v Reaper Paints]
Colour Comparison Charts
Paint Range Compatibility Chart
Vallejo - Citadel Paints Comparison Guide
Citadel to Vallejo color conversion table
Note: My personal choice is Vallejo Game Colour (VGC) range and Vallejo Model Colour (VMC) range, but I do own and use other brands like Citadel and Derivan.