There is a 300% chance that your horse looks absolutely adorable fully braided or au naturel, so I don't think there is a wrong answer here.
If your horse gets braided at every show and you absolutely adore the way they look all dolled up, then absolutely hire a braider! If you prefer their mane flowing and free, then go with that!
Braids will create a neater, more polished look - especially for black backgrounds, but that doesn't mean it's the right answer for you.
Something else to consider is how your horse feels about braids... are they extremely tolerant and don't mind standing still to get braided? Or do they act like a full blown dragon & it's an extremely stressful process? Braids can take an hour or more depening on your horse & your braider, so factor that time in & how cooperative you think your horse will be after having their mane done!
I like to think of it like this: when you look back at these images in 20 years, are you going to be happier if your horse was all dolled up for the occasion or are you going to wish they had been in their natural state? Go with that answer!