IF, you have a Korg PA...then you could nicely add the SD-2 or SD-4 for adding more/different voices, especially lead voices. OAND/OR you can add PC baed sounds using soft synths (East West Golath as example)
With the Korg...you can setup a user voice slot with external mode...specifying the full MIDI bank/program change data and saving it. Then, when you select a performance, OTS etc. (or even voices in a styles using user sounds) the external module will work beautifully!
I do it all the time. Its a beautiful thing.
When I had my T2, I called Yamaha support to ask about how to use a external module(how to do bank/program change) and I WAS TOLD...NO Can't do it and I DID'NT NEED THAT!
I guess that's one difference between a PRO arranger and a home keyboard. Yamaha markets as home keyboards and that's fine by me...unless you want the PRO features not in a home keyboard. Sold the T2...now have a PA2XPRO.
I sure liked a lot about the Yamaha...but I wanted some of the features in the Korg more.
Before you buy any of them...spend he time to see which one you like the sound of and which features are the most important to you!