After talking to people who have had both types--the single units work much better than the combos, + you can wash and dry at same time. And both the washing and drying take less time in the singles.
From personal experience with Whirlpool and now Spendide stackables, my wife says she would opt for the Whirlpools every time + they are usually less cost.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired