I think sometimes "Grass fed" is a bit of a sneaky sales ploy - in the UK most of our cows will be grass fed anyway - AFAIK it's mainly USA that this may not always be so. Just look at plain ordinary 100% butter and not spreadable or light(er) versions which may have non-butter additives in the way of oils, water, etc. Check nutritional information if in doubt.
Salted or unsalted is up to you.
My preference has always been Kerrygold salted - for almost as long as I've been buying butter, and since well before I was ever diagnosed with T2!