Mike,
Given the choice, I would not opt for the power rack. With some cars, like the 16v Golf, you don't have a choice. Whichever rack you choose, presumably it will go in before the engine.
I've run a welded diff in a Rabbit for 5 seasons and see no need for power assist. At speed, the steering is not heavy, although you do get more torque steer in throttle transitions. Around the paddock you want to feel the resistance so you don't overturn the wheel and break your CV joints. That 90 degree paddock turn that racer-tim mentioned is usually done in three incremental turns unless both front wheels are on the grass.
Joel