Commercial farms are great placements to have, as health of the animals concerned is arguably more important if animals/animal products are entering the food chain, but urban farms are a great way to get your foot in the door!
Obviously all farms will be different, but personally I have learnt loads from my urban farm placement. The farmers have taught me about a myriad of diseases that cows can get (mastitis is the first one that comes to mind, but they have both imparted their knowledge on acidosis, ketosis, several different types of worms... to name but a few). In addition to this, I have had more hands on experience than I could have ever dreamed of, I didn't expect to be allowed to milk my cows on only my second say there!
I'm so confident now that the farmer will just tell me to do it while he watches, I've got it pretty much fathomed out my now! Urban farms tend to have a wider variety of animals than commercial farms. Mine for example has several breeds of sheep, cows, goats, pigs, geese, chickens, turkeys, ducks, rabbits, guinea pigs, horses, fancy pigeons, guinea fowl... the list goes on. Being able to work with such a large collection of animals is honestly so fascinating, no two days are the same! There is always something going on that you can observe/do yourself... of course it's what universities are looking for!
When phoning up for my dairy work I mentioned my experience at the urban farm, which helped me to secure a place. Even if it's only for a week or two, urban farms are utterly brilliant in every single way.
Sorry, I have rambled.
tl;dr: Personally my urban farm placement has been my favourite one so far,
and in my opinion you'd be mad not to try and get some work on one yourself