Old Bill Dibble said:
Better trigger and a standard night sight. I think the receiver has a different treatment as well.
Only the original SD models introduced in 2010, with no "VE" suffix, had the tritium insert. These pistols were dropped from the catalog in 2012 when the SD9 VE replaced the SW9VE. (Note the space before the "VE" on the newer pistol.)
IMHO the major advantages of the SD series over the previous SW series are (a) dovetailed M&P-type sights, which provide plentiful replacement options; (b) the improved trigger; and (c) a standard Picatinny-style front accessory rail rather than a proprietary rail.
Here's a more thorough rundown.
The SW9VE had a proprietary straight accessory rail, less elaborate slide machining, different grip shape and texture, no frame "fingerprint", and a longer-travel and slightly stiffer trigger, and it takes different magazines. The sights are unique to this series of pistol, and the factory front sight cannot be adjusted or removed non-destructively because it's attached with a plastic peg that is melted or "mushroomed" into place; the peg must be cut and the stock front sight discarded if you want a front night sight. Several different colors and finishes were optional, but the basic SW9VE had a satin stainless slide and black frame.
The SD9 was introduced in 2010. This pistol has fancier slide machining with front slide serrations, a revised Picatinny-style accessory rail, revised grip frame shape and texture, a frame "fingerprint" for your index finger when it's off the trigger, redesigned magazines, M&P-style sights with a front tritium night-sight insert, and a slightly lighter trigger pull. Slide finish was black Melonite and the frames were black.
In 2012, S&W dropped both the SD9 (non-VE) and SW9VE and replaced them with the present SD9 VE, which carries over the SD9 non-VE changes, but with satin stainless slide finish and no front tritium insert.