Hi, flexible Tanks have just one advantage: Access trough a small
hatch or so, can expand in there and therefore more Volume in a dedicated, inaccessible space. Thats it - If you can fit rigid tanks and you would be happy withe the volume its a no brainer: safer (no chafe), longer life, mostly cheaper, possible to clean, better ventilation (a big thing with black water), better to secure agains movement and many more I guess.