The Airstream will pull easier (less drag), last longer, and hold its value longer.
That being said, I will agree with others that you may not be happy towing either with the 4Runner. You'll be maxed for sure. It might be ok for occasional shorter trips on flat lands but long, extended, or mountain trips would not be fun.
The 4.0l is a great engine but the gearing isn't ideal for towing. My old '04 T4R struggled to maintain highway speeds towing my 19' hybrid (3800lbs). Mind you it had slightly less power than the new ones and only a 4 speed tranny. Mated with the newer 5 speed may fair better but I'll bet you'd have to keep it locked in 4th to keep the engine from bogging down. After 1 season I traded for a V8 T4R which was awesome but unfortunately no longer an option.
If you plan/need to use a WD hitch (and you probably should with either tt) you'll need to upgrade to a frame mounted hitch. The factory receiver is only mounted to the x-member and is not rated for use with a WD hitch.