This is the real question of U/W photography when it comes down to brass tacks. How much of a pain it the ass are you willing to put up with for improved image quality? This question ranges from the micro 4/3rds cameras up to the Scarlet systems. Smaller is easier and you can argue that it is "just almost about as good". Bigger is ( so much harder to carry and pack) but slightly better. Logistics is the task-master of underwater photography. Do your put all of your photo gear in carry-on, or do you tell your wife that you are checking 200 pounds of photo gear, and she can not bring her entire shoe collection?
You knew the answer to this question before you asked it. A huge mega dome WILL make a difference in image quality. A big friggin' dome always has better optics than a smaller dome. The question is "is it worth the trouble?" It is impossible to answer that question for you! Can you take an increadibly good image with a smaller dome? YES!, Would the same image taken with a larger dome probably be a little bit better? YES! Is a bigger dome a lot bigger pain in the ass to travel and free-dive with? YES!
The other posts above have great merit, but you really need to decide what lengths you want to go to, in order to get the best possible image.