Both Maglite and Streamlight make excellent products. The Streamlight has made it's name on the rechargeable feature which is excellent if you use a light a lot.
In the past, instead of the "D" cell units, I've preferred the "C" cell units for two reasons.
First is that the can be carried in a standard police baton ring. Thus you can have a flashlight with 3-5 cells and a spacer, giving you an aluminum "nightstick" and flashlight in one.
Second is that the smaller diameter "C" cell units are easier to hold in the hand and use like a baton than the "D" cell units. This means retention if you're using it defensively.
I don't know if either Maglite or Streamlight sell the "baton" style lights any more, however. I was used to using the original "Tac" light made by Kel-Lite