The only way to really compare the responses of the FET and Tube preamps is on one of the heads that have both, such as the GBE's or the Max heads. The reason for that is because you'll be going through the same power section so any differences will be related to the preamp only. The Shuttle 9.0 is different from the Shuttle 6.0 even though they both have a tube front-end.
Having said that, to my ear the differences are very minimal and you have to do a direct back-and-forth between the two channels to really appreciate the difference. But in general, the FET channel is a little quicker. It's a bit snappier up front, and the notes are uber clean.
By comparison, the Tube channel is a bit more relaxed up front, and the notes have a bit more warmth to them. This is true even when you try not to push the tube hard. If you started with the tube channel, you might think it was very fast and clean, and then you switch over to the FET side and it's like, "Oh, now that's what uber clean is!"
They both sound great to my ear, and neither is a make or break kind of thing when it comes to getting a clean and fast tone. The Tube preamp gives you the ability to push the pre a little hard so that the notes get warmer and less articulate, even before things get gritty. So you could argue that it is the more flexible of the two since it still it can go in two different directions.
The FET is what it is. Always clean and pristine, to the max. If you try to push it, it'll stay clean until it starts clipping, and it's a bad sounding kind of clip. You can't overdrive it.
Hope that helps!