If over 20 you don't decide either, you get both.
The question isn't which is better so you can make an election choice. The question is how much money will you receive you so you attempt to build a budget.
http://www.benefits.va.gov/COMPENSATION/resources_comp01.asp shows the VA compensation rates. How much they pay at 90% is dependent on if you're married and the number and ages of your children.
Whichever is higher, the 50% retirement (which depends on your base pay) or 90% VA disability compensation (which depends on # of dependents), that is how much money you will receive.
So scenarios:
Base pay is 4000, married, no kids
DFAS pay is 2000, VA pay is just under 1900. You will receive ~$1900 from the VA, untaxed and $100 from DFAS that is taxed.
Base pay is 2000, married, 2 kids under 18.
DFAS pay is 1000, VA pay is just under 2100. You will receive ~$2100 from the VA untaxed and nothing from DFAS.
In neither scenario does it make sense to elect for DoD money, as all of the DFAS pay is taxed. Waiving retired pay for VA pay doesn't negate your eligibility for what is left over of the DFAS money. There is never a reason I can imagine to not waive retired pay for VA pay.