I guess I should ask, what machine, and are these your ONLY options?
Trashy?
neither.
I use a 4x6 or a 5.3 concentric on the DFX with the EEPROM hi trash program, or MXT 5.3 with low gain and his disrim but I digress...
Smaller is GENERALLY better in trash, but that depends.
The DD WILL give you better seperability in trash, but not a HUGE gain over the concentric in the size you mentioned.
Of the two you mentioned, the DD is better at handling trash, but the larger coil size MIGHT nullify the advantage..
In my experience, they should be about even, but "trash' is a difficult thing to gauge..