Hi Kelsey,
MCID and MDC are different things that serve different purposes. Often, both are not available for a specific test. If both are available, I tend to favor the MCID because it inherently has clinical significance versus the MDC that is purely a statistical measure.
More importantly for this question, you want to make sure you are comparing apples to apples. A significant change measure for gait speed (usually the 10MWT) does not necessarily apply to a significant change on a different test (the Six-Minute Walk Test). You’ll want to use significant values for the 6MWT for goal setting for the 6MWT. Similarly, you’ll want to apply significant values for the 10MWT to goal setting for the 10MWT.