Kind of already been mentioned...but I think both are very important. Working the flag, no matter how inexperienced or experienced your horse is, is a great way to fine tune those little things that your horse may be having issues with on the cow. You can slow it up as much as you need to, while when your on a cow everything is much more fast paced, and it may be harder to stop and correct your horse. Try making sure its right on the flag first, then see if they improve those movements when working a cow. We have a 2 year old in training right now, she definitely started out on the flag, then once she started to get a general idea, we started bringing her to work cows every saturday, but instead of throwing her right in with a herd, we just put one cow in the arena with her and let her work it off the wall. It's a great way to ease a younger horse into it without pushing them or overwhelming them too much right at first. We don't have cattle over the winter though, so are just utilizing the flag 3 days a week until February when we have cows again... (it can't come soon enough, that flag sure gets a little boring!!)