The "ISO Standard" argument against inflation is that it favors the capital class over the middle and lower class, because money loses value over time faster than capital does. This means it steals from the poor and gives to the rich, because the upper class puts most of their wealth into capital, because they have a higher return on investment, while the poor and middle class store most of their wealth in savings accounts, because they need the ability to access significant chunks of their wealth in a hurry if they suffer an emergency (the rich have "lots" of cash on hand, too, but it is a smaller fraction of their overall wealth).

This seems like it implies that a deflationary currency would be better, but it does not, because in a deflationary economy, the behavior of the rich would change. In effect, the rich would keep most of their wealth in cash, while the middle class and the poor would not be able to change their behavior at all. The middle class family that has 50% of their wealth tied up in a house would still have that wealth tied up in a house because they actually need to live in it. A rich investor who owns hundreds of houses in the current economy would, however, move their wealth into cash and sell the houses. It might help the housing market, but it would screw up the currency market in exchange, and would probably not do anything for wealth disparity.

In short, "wealth" is the ability to make a wider range of choices. Both paths, deflation and inflation, favor the wealthy over the poor because of this.