I thought you meant "in that choice" as in between the HDC4 and the DD, just a small misunderstanding. Also, for this "exact setup" (one SQ2200) I believe the DDs would be superior - perhaps not in the long run if he added a second amp. But then he runs into the problem of bridging the amps at 1 ohm - which gives a load of 0.5 ohm to each amp and would be highly taxing on his electrical system. I would want to get subs with a different voice coil configuration from what he needs now and shoot for a final load of 2 ohms bridged if it was a daily driver - maybe even upgrade to a SQ3500 or similar instead of buying a second amp - but that's just me.
If it were my vehicle, I would choose neither and upgrade subs when the larger amp came. There isn't really one sub that will perform better in both scenarios from what he gave as options. Something a bit beefier than the DD without moving up to a less efficient 4" coil might be ideal for both, though