The main difference is Write speeds and the flash being used.
Write speeds are self explanatory, the Pro gets around 450MB/s while the Vanilla 840 gets around 160MB/s. Read speeds are about the same at 450MB/s.
The flash used in the Pro is MLC, while whats used in the Vanilla 840 is cheaper TLC. The MLC is what lets the Pro have a better write speed, and also stands up to far more rewrite cycles before it begins to degrade. However I wouldn't be concerned about that, any modern SSD will last well past its obsolescence before it starts to degrade from normal usage.
So basically, do you need better write performance? If your just using it as a boot/programs drive, then IMO you dont.