The MAC 239 is a long time favorite of mine. It is a flat shader brush made of white goat hair and is best used with powder products.
The MAC 242 is also a flat shader but made of synthetic bristles. Because it is synthetic, the 242 can work with powder, cream, and liquid products.
The bristles of the 242 are slightly longer and the 239 is wider.
I did a little experiment the last couple days to see if I could get an identical result using the 239 on my left eye, and the 242 on my right. The eyeshadow I chose to use yesterday is Club -- and if you own this one, you probably know why I chose it. Club is somewhat of a tempermental eyeshadow and if you want its duochrome to show it really has to be packed on. I also tried this with Saddle (Matte) and All That Glitters (Veluxe Pearl).
My results: The 242 without a doubt packs on more color, especially when using Matte and Satin eyeshadows. Because it is made of goat hair, the 239 eats up more color and doesn't deposit as much on the eye. Although the 242 deposits more color, I can't say it is the clear cut winner. The 239 gives a softer look -- so I guess it depends on your preference. I think the 239 is better for beginners and the 242 is a great addition to any eyeshadow addict's brush collection!
Do you have the 239 or 242? What's your favorite brush to pack on color?