Dear JPO,
TWAIN is a standard used to regulate the communication between the computer (and its software) and your scanner, you can picture that as a connector. Further information about TWAIN can be found here:
http://en.wikipedia.org/wiki/TWAINor here:
http://www.twain.org/The difference between the TWAIN version and the standalone/photoshop version is:
The TWAIN version of SilverFast needs a host program to be started (it means that it will not be listed in your program's list) and uses the TWAIN standard for communication between the scanner and your computer program. There is no color management system in the twain version, the color management will be done afterwards in the program you use as a host program or later in an editing application.
The standalone/photoshop version works as a standalone application or can be called from photoshop without the use of the TWAIN standard. This version does not need a host program, it means that you can start SilverFast without any other application. Or if you preffer, you can also call SilverFast directly within photoshop.
The standalone version offers a color management system.
There is no furhter difference between this two versions, if you want to produce JPG files is irrelevant if you use the twain or the Standalone verison, the end file will behave the same way.
Thanks for your interest
Have a nice day