I think a very important question here is: What is your expierence level with snakes? Have you kept large constrictors before? Can you afford to house a 20 foot snake??? My cages (before the cost of heating) cost WAY more then my snakes did.
Also, who told you retics where more "trustworthy"? In my expierence with them, retics are WAY less trustworthy. Unpredictable to an extant. Where as most burms just lounge around and don't do much. But they get heavier and more powerfull. So if they do attack, they can screw you up pretty bad. Both can though, don't forget that.
Also, do you have someone that isn't afraid of snakes that can assist you with cleaning and handling once the snake is to large to safely handle your self?????
Just some things to think about if you've never owned a big snake.