Which torque converter for my car?

You pick the stall speed based on what the intended use of the car is and what engine modifications have been done. The reason being the higher the stall speed the more heat it generates from slippage below that RPM, that can make it a poor vehicle on the street as a daily driver. On the flip side if you are building it for race or strip use you want the higher stall speed to allow the engine to build rpm before it fully engages. If I remember right the stock unit is around 2,000 RPMs. That works fine for all around use with the 3.8.

With it modified you may need to keep the engine spinning a bit faster to avoid low end power losses. Is that case I would go for the 21-25 and install a large trans cooler to help lower the fluid temperatures. Even dual coolers with fans depending on where you are located. That way you can still have decent drivability around town without bogging. A transmission temperature gauge would be a good addition as well, just to keep an eye on things.