Putting it at its simplest, Jim, the bigger the battery, the more electricity there is within it. D cells are the same voltage as AAs, but that voltage will last much longer. Overall, D cells may be cheaper.
As to rechargeables, a rechargable D cell doesn't hold as much as a "disposable" one. Same with AAs. And rechargeables of any size only put out 1.2 volts each - 80% less than their disposable equivalents. That means that if your torch really wants 6 volts from 4 x 1.5V batteries, it will only get 4.8V from 4 rechargeables. The beam won't be anything like as bright. Of course, if your converter will take five 1.2V rechargeables, they will add up to 6V.
If you fancy your chances
![:zap:](https://madmodder.net/Smileys/default/electricf.gif)
at simple electronics, "disposable" batteries can be recharged ten times or so - see
http://madmodder.net/index.php?topic=2583.0 Andy