The customization points aren't much in the grand scheme of things, 15/15 is hardly gimping yourself. My WHM's set at 20 MND/10 VIT and I do perfectly fine, I'm considering going 15/15.

As you get higher level and gear gets stronger it really starts overtaking the custom points and by the time you get to stuff that requires crazy gear, your gear will more than cover what you need. Obviously if you end up liking more than the other you will want to focus on that one, but if you want to and have the opportunity to play both you can split just fine.