One advantage of the X Max or X Pro is that the lower shell is heat moldable - that is to say, the boot will mold and adapt to your foot within certain parameters. The liner is also heat moldable.
Regardless, you should try on any boot that you are considering purchasing. Take advantage of the bootfitter's knowledge of what he sells. Ordering boots online is like getting a mail order bride. Without the boot on your foot, you are shooting in the dark.
I purchased the X Max 100 this year, but I tried on the X Pro also. The X Pro fits a bit wider. (The range for the X Max is 98 to 104, the x Pro is 100 to 106 mm. That is the width of your forefoot) The bootfitter and I both agreed that the X Max was likely the better choice for me out of the box, although we heat molded both the liner and shell.
You should also consider the purchase of a custom footbed to give your flat feet the support they need. A hundred dollar "custom" footbed is a waste of money. You may spend $250 to $300 or more for a really good footbed from someone who really knows what they are doing. But it can be transferred from boot to boot and may last you 10 years. Also alignment and canting as needed. It is attention to these kinds of details that really boost the performance of a boot and, most importantly, your skiing. If you have never had any of these done, you don't know what you don't know, and you have not experienced how good your boot can be.
Talk to your boot fitter about what stiffness would be right for you. Be realistic about your abilities with him (he wants you to get the right boot) and be realistic about your leg strength. If you cannot flex the boot deeply whenever you want to, fast or slow, then you have too much boot. A softer boot that you can flex well will perform better for you than a boot that you cannot flex readily. It took me a while to learn that. I ski much better in a 100 flex than a 120, I've had both.