No difference- the fatty-acid content of beef suet is not dependent on diet, since the osmotic exchange of fatty acids into and out of adipose tissue is very small.

Omega-3 is animal in origin thus not a component of an ox's diet, vegetables have omega-6, which has never been high on anyone's list for enhancing health. Omega -3 is produced in an animal's body, it is one of the unsats an animal needs- your own body can and will make it also. Today's paper had an article citing a study in the UK which unexpectedly showed NO significant relationship between omega-3 and heart disease. We know suet has plenty, so what's up?