daebis

Hi, your inbox is full, so I'll give you an answer to your last PM here:
I just checked in WinISD:
With both a subsonic filter (16Hz 18dB/oct) and a low-pass filter (63Hz 24dB/oct) applied, the HF needs about 30 liters to give the same curve as the HO in only 20 liters...

But to reach the same volume level, the HO takes more than 1.5 times the power of the HF...

The HF is also a better choice if you don't want to apply a subsonic filter...

Conclusion:
If space is an issue and power isn't, I'd say the HO is the better choice.
If you have no problem to give each sub at least 30 liters, I would prefer the HF