If the WDH with sway control costs about the same, you can't lose by adding it.
A lot of different things can contribute to trailer sway: trailer tire design, trailer loading, suspension, road condition, cross wind, other traffic (like semis), etc. Some owners never experience fishtailing, and for others it seems to a chronic problem. Hard to predict.
A pure WDH will eliminate some of the physics that can cause sway, but not entirely eliminate the possibility. A sway control bar will pretty much dampen the remaining forces and eliminate any potential for fishtailing. Note however, that if you have severely unevenly inflated tires, or your WDH isn't properly set-up, or your trailer loading is way out of whack, even a sway control bar might not work!
1967 17' Hi-Lo Bon Voyage (1967-1969)
1969 19' Hi-Lo Bon Voyage (1969-2011)
2004 27' Hi-Lo TowLite (2010-present)
2015 Chevy Tahoe LTZ