We had Dexters here when I arrived. I would much prefer a small Jersey to a Dexter. The Dexter is a suckler cow you can milk if you get the right one and train her right. The Jersey is a dairy cow. Chalk and cheese.
Dairy cows love routine, and are very trainable therefore, pretty placid and biddable.
A lot of smallholders will want the cow to rear her own calf, which of course a suckler cow can do - but will she let you have milk as well? Probably not, not after a certain point, maybe two months in if you are lucky. Whereas a Jersey is easily trained that you will leave one quarter for her calf so she can let you have the entire contents of the other three.
A Dexter calf is not a lot bigger than a large dog at 9-10 months old, still very teenager-y in shape, really needs two summers to make a carcase worth the butchering. Whereas a beef bull on a Jersey cow gives a fine beast at 10 months old, if it suits your system better to send them off at that point, as you dry the mother off for her next calf. And by definition, smallholders are often tight on acreage, so sending the calves off at less than 12 months old is real bonus.
I guess you would maybe want the semen from one of the original population (smaller) Angus or Hereford bulls for a miniature Jersey, but I would imagine the growth rate and maturing rate would be similar to the full-size story.