mixed breeds are certainly good for some situations, but preserving established breeds deserve an equal amount of respect and support...
also be sure no matter what your choice its what you WANT, i personaly belive no matter what your goals, milk, meat, pelt ect....you need to enjoy what your raising or it becomes a chore...
for me that means focusing on breeds i like that will give me what i want...
rabbits for example...im going to be raising rabbits for meat and pelts...now most people would say go cali or new zealand, they give the best growth rate, large litters ect...but quite frankly i dont LIKE them all that much and if i have to feed and rasie them i want something i enjoy too so ive decided to go with the standard rex...a breed i enjoy, they do make good meaties ect...they just not as commonly used.
i think this applies to all animals.
you want eggs your best bet is productin layer hens...but what if you dont like them?!
but to me this is the difference between comercial farming (raising things for a profit) and homesteading, when your homesteading its YOU that has to do it, you cant relay hire a team of folks to do all the hard work, and in most cases you have to look at and interact with it every day.
so if you dont feel comfortable handling a longhorn bull, thats certianly not going to be the breed for you even if it fits every other way possible into what your looking to do.
im also a big fan of preserving heritidge and rare breeds that have fallen from favor for no other reason than someone developed a breed that grows faster...
(ie the golstershire Old spot pig...the asolute perfect homestead pig that makes the most amazing bacon and lard...yet there hard to find because they grow slower, and take a little more to get them to the table...not "efficient" when were talking fast turnover...but potentially beter for the situation.
a mix might be absolutly perfect for your situation...
but so could a pure breed.
(youll also make more oney on live sales on pure bred "rare" animals is you can get into that side of the market...)
efficiency might also be something to cnsider...a cow no matter what its breed or how fast it grows or how delicious the beef, if it doesnt do well in your climate, or gets fungal infections if its on too much wet ground and you like in a wet area...or a breed that cant stand the heat and you live in the middle of the desert ect...
match your wants, your needs, AND your situation
for me its the same as buying a husky when you live in mexico lol...sure you can baby it so itll do ok in that kind of weather, but the dogs not gonna thrive, your not going to be happy and your going to have to put in alot more work. livestock is no different.