If you're movin on up, I wouldn't discount Tahoe/suburbans either.
Right now in most parts of the country, big gas guzzling SUV S are selling for a premium due to cheap fuel, good economy and lack of inventory due to the slow down few years back.
$20k for a great shape lower mile full size SUV will be tough I think. I'd keep my eyes peeled hard for the right one and be ready to grab it.
Also, both brands are much improved around 2011 when the went to 6 speed transmissions. I'd pay less for a comparable 4 speed rig. Although what you're towing will be fine either way.