Hi. Sorry I didn't reply last night.
Shag is pretty loose and lays down easily, and it has an almost matted appearance when it's like new. I think because of that, it tends to have a high Performance Appearance Rating (PAR).
Friezes sometimes have a fairly low density but not as low as shag. They also have a high twist and that helps with the PAR, and they also sometimes have the flecks of color that some shags and most berbers have. Friezes, shags, and berbers are all dependent on personal choice -- either you like them or you don't.
It's just me, but my personal preferences lean more toward the cut piles -- textures and plushes -- and not so much toward the berbers and shags. I do like some (but not all) of the friezes. Plushes or saxonys have a smooth velvety appearance and do show foot prints and vacuum tracks more than textures, which have a knobby appearance that helps hide them.
Like I mentioned before, if you want one type that's soft and cushy for the bedrooms and another type that's a higher density for the hall and stairs, believe me a lot of people do that.