Four factors decide it: weight, activity, budget and expectations. FHO, at $3,500 to $5,500, reliably relieves pain and suits light, well muscled dogs, with a functional but slightly imperfect gait. THR, at $10,000 to $16,000 per hip, rebuilds near normal function and is the stronger answer for heavy dogs, athletes and owners who want the ceiling, at specialist prices and specialist stakes.
The rough weight line surgeons talk about sits around 20 to 25kg: below it FHO results are consistently good; above it the compromise grows with the dog. Activity matters as much: a working dog or runner uses the extension an FHO hip gives up, while a companion ambler may never notice.
The classic dilemma is the large young dog with bad hips and an FHO budget. There is no universal answer, but there is a right answer for your dog, and it is readable from the x-rays, the dog's build and your actual numbers. This exact fork is one of the most common cases we referee, precisely because the price difference is so large that a $250 independent read is trivial against it.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.