Overall reliability would be better, as the usual situation in case of failure of one of the Marston Vale units is that the buses come out until the end of the day when the unit can be either fixed or swapped. Whereas with 3 units there would be a spare at Bletchley.
If one was under maintenance, they would know about the issue and perhaps could pre-warn passengers of likely issues. And as the 230s are effectively new they *should* be more reliable.
As for 153s on the Stourbridge, I've not heard of that happening. I have heard of the odd bustitution, though.