Liquid is probably purer; the drying process can cause some mutations of the yeast. That being said, there is a lot more yeast in a pack of dry than in a liquid yeast pack, so you probably get more 'pure' yeast.
There is a lot more variety of yeast with liquid. Again, it is to do with the difficulties of maintaining the yeast during the drying process.
Unless there is a particular yeast property you want that cannot be found in dry, I would recommend using dry.
S-05 is a good clean American, supposed to be the same as 1056.
S-04 is a good British yeast.
Nottingham is a good British yeast with good attenuation and low temperature capability.
Outside of these, I would seriously consider liquid. Windsor, Muntons, Coopers, all seem to have issues occasionally. However, I exclusively use liquid these days, and do not have much experience with many of the dry yeasts. I'm sure others will chime in with their comments on these yeasts and flame me for putting down most dry yeasts without really trying them.