How to create a single measure that dynamically switches between showing a number or a percentage..

BarbaraYanChen 
Easiest way to create a separate disconneted table for slicer and create a combined measure for this.

Combined Measure = 
IF(
    SELECTEDVALUE( SlicerTable[Sort] )= 1,
    [MomValue],
IF(
    SELECTEDVALUE( SlicerTable[Sort] ) = 2,
    [MomPct],[MomValue]
) )

For Percntage formatting use measure's Dynamic format option and use below measure

IF(
    SELECTEDVALUE( SlicerTable[Sort] )= 1,
    "#,##0",
IF(
    SELECTEDVALUE( SlicerTable[Sort] ) =2,
    "0%","#,##0"
) )

Below screenshot

But, if you want to dare to see your screenshot result then create calculated table and use below code ( ideal way to create this table in Power query by simply unpivoting).

Sctsht = 
VAR _1 = 
  SUMMARIZE( 'Table','Table'[Month] )
 VAR _2 = 
 UNION(
  ADDCOLUMNS(
	_1,"Value and Pct",[MomValue],
	"Tag","MomValue","Sort",1,"MonthSort",IF( 'Table'[Month] = "JAN",1, IF( 'Table'[Month] = "FEB",2 ) )
  ),
  ADDCOLUMNS(
	_1,"Value and Pct",[MomPct],"Tag","MomPct","Sort",2,"MonthSort",IF( 'Table'[Month] = "JAN",1, IF( 'Table'[Month] = "FEB",2 ) )
  ) )
  
RETURN
_2

For fomatting use same code.

IF(
    SELECTEDVALUE( Sctsht[Tag] )= "MomValue",
    "#,##0",
IF(
    SELECTEDVALUE( Sctsht[Tag] ) ="MomPct",
    "0%","#,##0"
) )

Below screenshot

Attached is the pbix file

https://we.tl/t-pCBQoFQDhz

Hope, it will help.

Regards

sanalytics