I don't prefer one formula over another. I have a lot of pressed and loose e/s that I love. I tend to have more loose than I do pressed. I definately adore MAC piggies. A lot of girls actually press their pigments and loose eyeshadows for less mess.
I like to use pressed shadows as shadow. I use loose to mix with glosses and also with a liquid that I have to make a liner. I have used them in polish before also.
i just bought a bunch of loose pigments but it really is messy, especially when it drops onto my cheeks >_< but i think loose pigments are much easier to blend together than pressed powders
i like loose too. just recently discovered them and boy o boy you get soooo much better pigmentation. you dont have to scratch your lid with a brush or sponge applicator. you just tap pigment on and you get lots of shadow right where you want. there is a bit of fallout though, i dont mind
If you're okay with a bit of fall out, I find that the pigments are just far beyond any colour that a pressed powder will give in a single application. It's so easy to get vibrant colours without having to pile on the layers. Plus they are killer for blending. Not as forgiving, though, so unless you feel the need to have very bright makeup, you're probably okay with pressed powders.
I think both have about the same pigmentation and durability, HOWEVER, as others have said, I find the loose ones to be waaaaaaaaaaaay too messy for my liking.