When you're making your own game, having just one illustrator do mockups for your art might work sometimes but using multiple illustrators means they approach the design from different directions which in turn gives you a wider range to find your perfect style for your game. Ideally you would have enough time to ask each illustrator to do multiple versions and mix up their style. That's sometimes when the magic comes out.
Don't forget to ask your target demographic what they think of all the versions of your game art. Ask them not only which one they prefer, but why they like it, why they don't like the others, and what they think can improve it. Listen. The best liked style for your game should then come to light.
