ISE wins out!
Most will know I am a devoted Kitchenaid fan but the ISE in the picture appears to be the unit that marked the transition to the Whirlpool Power Module pump. This is an excellent washing system and on this unit, it did have the heat delay during the main wash portions of the full cycles. That is, it would heat the wash water to approx 140 degrees and the timer would not advance until it reached that temperature.
With today's enzyme based detergents as opposed to what was available when the Hobart/Whirlpool 21 series were being built, you may see better results with the ISE machine. It also has the Rapid Advance timer and no dial to turn as on the Custom 21. The ISE is nearly 100% identical to the Kitchenaid 23 series machines.
As mentioned before, the timer and controls differ but more significantly, the pumps cannot be swapped between the machines as the power module uses a smaller opening in the sump so no advantage to having one as a spare for the other. All the other parts for the most part will swap out.
I would grab the ISE and try it out. I think you will not be disappointed at all.
That is not to say that the Custom won't perform nearly as well. Just the bells and whistles have to count for something.