maby build a flat down and just build the other piece so it can be put on the front as a down or the back as a flat(an option thing) then you can have both...if not i like fdf's better
i prefer down flat downs too but thats just my opinion. i say go with what you like more. but, you do need a pretty steep incline for a d-f-d or else you'll have to make it extremely tall and it would probably just end up sucking. good luck
if u have a hill near your house you could use i would def build a fdf. i built a 24 foot fdf box that is soooo fun. and the best part is that when you want to change it up you can just put it up on a hill and it becomes a pretty mellow dfd
or a FdF but make it so you can switch up the lenghts of the supports it could be a DFD or a FDF. Cuz if you think about it a DFD is just a FUF (flat up flat) with different heights for the supports