For a kitchen (or bath) i'd likely use cotton (LB kitchen cotton/bernat's crafters cotton/sugar n cream/peaches n cream)
I have made bath mats, using 2 strands held together, and while i thought is was a stupid idea at first (all that work to walk on!) i must say, i love my knit bathmats! they are so comfy under foot (i am likely to make my self some more.. (i did make 2 for my DD--one perfectly matches her shower curtain!-by sheer luck!) and she too is impressed at how comfy they are under foot.)
I have thought about (and one day yet might do it) making a wool area rug.. knit loose and over sized, and then full/felted down to a smaller size.. but i wouldn't make a wool rug for a kitchen.
kitchen rugs/mats get dirty quickly and need to be washed often.. cotton is the way to go i think!
My co-worker was telling me about her latest knitted bath mat craze, and she usually holds 4 (4!!) strands of cotton together and does a bobble type stitch to create some texture. She says it makes awesome bath mats.
I haven't made bath mats myself yet, but if I did I'd go cotton as well.