In my opinion, I would go with the one with the most towing capacity.
What are the overall towing numbers for each vehicle? How do they compare? Things like wheelbase, trans
axle ratio,
weight, horsepower, etc. Are they both 6 cylinders? I don't know much about the Outback, but I know at one time the Highlander could have a 4 or 6. I personally wouldn't want to tow with a four.
Look at the numbers and then decide which one best suits your needs.