In this image (source: Spade (Wikipedia), creative commons), the spades have a T handle. On most modern spades there is a D handle. There is of course a third alternative of no cross piece at all, just as with long-handled spades. Forks and other tools also use the two designs. So the questions arise: why would the different styles have arisen? Is one style better for a particular purpose and would we benefit from one of each in the garden shed? What about the ergonomics of the two designs? Which is most appropriate for a historic garden in say the 1800s? The other end of the tool (the blade?) is clearly a different issue; my question here is about the handle and its connection to the shaft.
A brief visit to some on-line historic tool collections indicates that all 3 types were common.
In my personal experience, the cross-handle (D or T, but D is more common in my tools, even the older ones) is more effective when shoveling from a loose pile, while the long-straight handle works better digging in raw ground - though either works for the other purpose, just not as well.
As one random example, I very rarely see long straight handles on manure forks, and have never, to my recollection, seen a D or T on a hay fork.
Grain shovels are almost always cross-handled.
As for D versus T, the T is probably simpler to make, while the D does not have the main shaft of the handle needing to be accommodated in between your fingers as you grip. A D of slotted and steambent construction is also arguably stronger .vs. twisting forces than a T.
The T and D handles are used for more controlled work versus a straight handle. The T handle is also used by those people whose hands don't fit comfortably in the D handle. I presume the T handle preceded the D as it's easier to manufacture but we'd have to go back well in history to be sure.
And an engraving of pruning tools as found in Jean de La Quintinie, The Compleat Gard’ner in 1693.