No idea about the Re Curve R6, but the 600 Lb would be the biggest you want. I think most here use the Fastway E2.  A 178 is a fairly light model, you should not have any problems. 

When choosing a wdh weight rating, bear in mind that it is lifting both the tongue of the trailer and whatever is in the rear of your tow vehicle. You don't want it bottoming out,  that will overstress it's tensioning bars and create some peak stresses on the trailer frame and TV hitch. 

Are you sure you need weight distribution? I do not with my SUV. It does not seem to need it. I do use an adjustable friction bar sway damper, just in case.

first and foremost...Happy New year to all.  i have always been somewhat (maybe alot) confused about weight distribution hitches.  I do understand that the intent is to distribute weight to the front for the intent of increased control,  does it also reduce sagging?  i know/am sure they work; just dont get how that works (Im not the brightest star in the sky so hope that makes sense).  I think (I may be wrong), but I believe I have read some podders use them on there f150's(mine was marketed as a work truck so it may have some beefed up springs) or other pickups.  not sure why a wdh would be needed as mine doesnt sag (maybe an inch or so).  I guess my thoughts are if your tv sags that much, it wouldnt be a proper tv and be unsafe.  im a pu guy and i completely understand thats not the vehical for everyone.  not being controversial (not sure if i spelled that right!!); its just i see alot of people pulling with scary rigs, and back to my original ?, dont understand wd.  sorry for the ramble.  jon

The tow rating of the '21 Highlander hasn't changed from my  2012. I ran a 1000 lb rated wdh on my 179 and the ride, braking, and handling was much better with than without. The 178 might be a bit lighter  on the tongue than my 179 was but I'm sure your rig would still benefit significantly from using the wdh. Note though that the purpose of a wdh is not to get the tow vehicle level, when set right the rear of your Highlander will still be lower than without the trailer. The objective of the wdh is to get the load on the front axle back to what it was without the trailer. You gauge that by measuring the ride height of the front axle of the Highlander without the trailer, then hang the trailer on it and tension up the wdh to obtain that same ride height. By getting that front axle load back where it was you are restoring the steering, traction,  and braking of your tow vehicle to what it was designed for.