They were actually all stainless steel. Stainless steel, however, is not rust proof. They had a tendency to rust at the sump and under the detergent dispenser. I can't remember whether or not it was a one piece tub. The machines were very well made; the door hinges were very sturdy, unlike many modern dishwashers, and the entire wash system was also stainless. Had one Waste King (1968 model) that rotted away at the sump and ruined the motor. Had a 1978 model that lasted about 12 years with no problems.
Were they better than KitchenAid? Hard to say, except for this - they did what a KA could do with no filtration system of any kind, in about half the time. The "Full Cycle" was all of 29 minutes (depending on model). That gave you two washes and four rinses. Their wash action was more on the turbulent side than the KA, not so gentle on finer china with wash temps that could easily etch glassware on the Steam Cycles (174 degrees). Lost a few fine china pieces and glasses in the WK. They were most likely more noisy than a KA Superba, and didn't offer a lot of overhead space on their non-adjustable top racks - only 6 inches. But you could load them any way you like, and they would clean just fine. I don't think, however, they lasted anywhere near as long as many KitchenAids did. Like any machine, there are trade-offs, but the WK's were completely unique and made such beautiful sounds that you'll never hear anywhere else. The one thing WK/Thermador had going for them that KA did not is that the earlier WK models were MUCH less expensive than KA. My 1978 SS878 was a whopping $278, compared to over $500 for a KA Superba of the same vintage. That's a big difference for its time.
If you ask me what my favorite was, I'd still say it was a KitchenAid Superba....KDS18. Overall, I think, a better machine.