I have used the HM packs quite a bit and find them very good batteries. I also use a fast field charger at the field if the batteries get too low. The one thing I have noticed over the years is the NIMs don't like the added heat from being charged too fast and wear out faster over time.
I haven't noticed any faster drain using the 6 volt battery pack but according to the experts it is something that happens. I check my batteries before each flight and if they are down I just stick them on the charger for a while.
If you happen to need some extra weight to CG then you could give the two battery pack set up a thought too, I have done that a number of times.
I still have some old chargers at home and they will only charge up to a limited mAh so it's something else to keep in mind, will your charger give them a full charge? I use SUB-Cs in my big planes and they are 4500mAh and my home chargers don't go up that high. I charge them at home and leave them on trickle then plug in my fast charger on the way out to the field. Not the correct way to do things but I haven't gotten a better charger yet so it works.