It depends.

The D gives me better positive control when doing straight digging.

The straight handle is more flexible in terms of positioning. E.g, if you need to break up hard soil, it is easier to thrust a straight handle up and down than a D. If you need to work the material with the side of the shovel, the straight handle is better. I also like a straight handle better when unloading material from the truck bed.

If you are only sticking the blade in the ground, stopping it in and moving the material, then the D is my preference. I think I use the D handle about %80-85. I just don't do alot of the maneuvering type shovel work.

personally i have about a dozen shovels of different makes.

long straight handles with flat blades and some with pointed blade. short straight handles of each. D handle of each. each one has it's own purpose in life. i keep a couple of each of the d handle and long handle pointed blades for regular use.

hoes

i have flat blade hoes, angled flat blade hoes, short handle hoes, and even pointed hoes. nothing beats a hoe for weeding around plants. what you are reading abotu tilling hte ground is jsut that, TILLING, not hoeing.

i also have a Garden Claw. it works good for my wife to weed her flower beds. also good for getting dandelions out of the ground. stick it down, give it a twist, and pull it up. it is hard to use in our clay, but in the beds themselves it works great. i also use it to mix up my compost.