Do you know the total weight and tongue weight of the Scamp? Actual weights, not published dry weights.
Start there. As mentioned, payload will be your enemy.
For 3-row SUVs, neither of those would be my my pick. The Atlas has a pretty mediocre powertrain, and the Ascent has a CVT. CVTs have a general inability to reject heat generated by the belt riding inside the pulleys, so it only takes a little low-speed-heavy-throttle driving to put it into fail-safe mode, and not much more to earn a replacement (Subaru has been very good about replacements, no questions asked). A traditional torque-converter automatic is still the most robust transmission for towing. Direct Injection Turbo motors make towing enjoyable, because they have lots of low-end torque and don't lost as much power with elevation.
Since you threw in the Rav-4... I'd suggest also looking at other 2-row SUVs, many of which offer more towing capacity and power.
