They all excel in different areas, are you looking at hardcore gaming and no caring about the heat of the card??
The Vapor X edition would keep your card cooler than the 3 and yet will perform at virtually the same framerates as the others.
Whereas the MSI 4GB edition will give you twice the VRAM, in future games. They might take advantage of more VRAM to use for the games. However it isn't the most important yet, and I haven't seen a Game need more than even 1GB of GDDR5
And the Boost edition of Sapphires is really just the factory overclocked edition. Sometimes it could mean nearly 10 FPS faster in games as well as faster FLOPS as well. Though most of the time it would mean it runs a lot hotter than its stock standard editions.
My suggestion is to get the Vapor X edition as it would run the coolest and even if you overclock it like it's Boost cousin, you would be running faster and cooler.