Several things you need to look at:
Brand reputation: Are they GMP certified, ISO 9000, full disclosure with customers, offer up certificates of analysis, accurate product claims...
Product Purity: Are there fillers? What is percentage of protein by weight? What is the product flavored with?
Ingredient Sourcing: Does the company discuss how they source material? What they look for in suppliers? Where the ingredients come from?
Company Values: Do they want to get the best product for customers or are they simply looking to draw you in on price? What matters to the company?
Do they support their products with good customer service?
And of course, the overall ingredient profile.
In the end, it does matter. Shit protein can cause some digestive issue and may be made with inferior ingredients. Also, some companies have been know to provide far less protein than the product label claims, so a responsible company matters.