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
, 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.