If you are planning on running the 300 lm siege on only 3 AA's you would probably be better off with the little UST Pico... or I guess there is a little siege?
You'll get higher lumens, and for a longer time with the little guy with 4AAs than you will from the big guy with only 3.
All of your chargers will take four cells at a time, so there is no time lost when charging either.
My tests with the Ozark Trail say that you'll get over 85 emitter lumens for six hours with 3 AAs (that's one for each D)
Personally I wouldn't want to top off 9 AAs each night of an outage to maintain maximum output levels of the lantern. If it will be a serious part of my outage lighting, IMO the only way to go is with two of them. Most of their run-time on alkys will be between 60-120 lumens, and two of them will make the room much more comfortable. If OTOH, I plan to use it regularly, then getting some NiMH Ds is definitely the way to go.
I am impressed by the long run-times offered by a set of 3Ds (provided that the demand is relatively low IE less than 100 lumens IE less than 270 ma. I'd expect that 3D NiMH batteries would deliver more than that easily for 5-7 hours, then start to sag. 3 cells are much easier to charge than 9.
regarding a charger for Ds, If finances are tight, or it is just tough to justify spending money on batteries AND a charger, you might use a charger that you already have and McGyver it to charge the D Cells.
I have two C cell nicads that periodically need to be charged. I cut two wooden dowels shorter than an AA battery, and put a small nail/tack into each end. I then use two jumper wires (with aligator clips) and a pair of little cylindrical magnets (that are really handy for all kinds of McGyvering)
to hold one end of each jumper wire to the end/s of the batteries. I then clip the pos and neg jumpers to the little nails, and insert (with proper polarity) the dowel into the charger. wheww. Yeah... I know.. what a PIA, but it was either make the adapters, or throw the NiCads out. I like being able to be flexible.
If one looks at the cost of a Tenergy D cell 10,000ma about $8 each. that's similar to an 18650 and they have about the same Watt hours. The D cell, maybe a little more. Three eneloops will have about 75% of the watt hours, and be somewhere in the same ball-park for cost.