Hopefully someone else here can give you a better idea for your area.
Might go to some of your local fairs and see what's being shown. Ask questions. Where did they get their animal. What have they fed them. Things like that. Every location is different. Would get some good information I'm sure.
Just a thought.
Theoretically, if the most value you put on the entire experience was the process, utilizing a Holstein steer as a show calf would be as valuable for the experience as using a traditional beef breed calf. I think you could be very competitive if your local show included a "Showmanship" category.
However, the terminal calf judging criteria usually should include the appearance of ability to produce beef. In beef production a Holstein steer calf will usually get hammered and probably finish last in any class that only includes healthy beef calves.
But all that being said, not everyone has the financial ability to purchase a good beef calf as their first show project. Everyone has to start somewhere. A dairy bottle bull calf might be a good place to start for someone that can't immediately raise the money to purchase a calf but can feed it. One other way that kids sometimes get the opportunity to show a calf they aren't able to purchase is to make a deal with the farmer. Some agreements I've seen include the farmer ensuring that the young person knows enough to actually care for and raise the calf. Also that the young person has the support of his parents, both emotionally and financially to feed and care for the calf. In some of those cases, the young person agrees to feed out a heifer, and is able to show it until it is possibly two years and two months old. When appropriate, the heifer is bred. After the young person is finished exhibiting the calf it is returned to the farmers herd. When the heifer weans its first calf, the newly weaned calf is given to the young person in return for the cost of the feed and the care he gave the original heifer.
You need to be a really good and serious young person for a good neighbor to trust you with a good calf like that though.
With regard to the actual breed of steer to use for the show. I would check at the show or with people that have exhibited in the show you intend to participate in. If they had both a Angus and a Cross bred or all other breeds class then I would go with the Angus. If they only had one class of steers where every type of steer went into that class then I would go with the cross breed black baldy calf. Cross breeds or hybrids typically out perform purebred steers due to "Hybrid Vigor". So if you were going to show against cross breeds you might as well have a cross breed to even the playing field.