I have always heard it is very hard to improve the udder, at least consistently. I have two does right now that are twins, born in the same litter. Their udders are similar, but also very different attachment wise.
I personally would say udder. Mainly because conformation is kind of you see what you get. Udder is hit and miss. The buck carries the unknown. In this sense I almost wish they did scorings on proven bucks, like they do for dairy bulls. That would be a great help!
I agree with the above posts, udders are tougher to improve than conformation. And I still can't predict what the udder on a first freshener is going to look like, even though I know what generations of udders looked like behind her! Teat placement and size - possibly, total udder attachment - not likely!
UDDER! Something so simple can ruin an udder and then you'll take years trying to breed it back to good again. Conformation is a lot easier to fix. My one buck's kids all have perfect feet & legs this year despite how the dam's feet & legs looked.
the highest expectation on udder improvement 27% from each parent the rest is pretty much a crap shoot. your best prediction for udders is the mother and paternal grandmother, if you know what they look like then you are 54% there.
conformation is easier to predict as you know what the buck looks like. i use a method called aAa adapted from the dairy cattle predictions for type. aAa is round sharp round meaning the doe is wide in the front sharp through the withers and round in the rear. ideally you would want aAA in the offspring so you would use aaA buck keeping the front wide, leaving the sharpness in there without adding to it as you want sharp not frail. and putting more angulation in the rear. add in the 54% that you can predict for udder and you can breed up in much faster time than the hit or miss method. if you know the milk production background this trait is 21% predictable. this is sort of a simplified formula, but it does work. i worked many years as a livestock geneticist. and now i have goats !!