Its all personal preference really. Mostly it will depend on the texture you're going for. Some FETs can sound really warm, or bright, depending on the mic. Tubes give a nice hi-fi sound but can be pushed into nice fuzzy distortion. The mic and pre can have transformers or not. Transformers can add color to the sound, which isn't a bad thing. FET preamps without transformer can be nice and transparent.
There's good and bad points to both. With a tube mic with a high iron transformer and a preamp with high iron transformers would normally be thought to really fatten and warm things up, but there can be too much iron in the signal chain.