What motors are in each? That would be my deciding factor.
There were 2 V8's available in teh Jeep, not sure if its the regular or long trailblazer, and that determines if a V8 was an option.
The 2 chasis's probably tow the same, i'd base my decision on motor, and tow ratings. Go with the more powerful motor, and/or the one with teh highest tow rating.
Neither are going to be ideal for towing. I highly recommend towing with a full size. And you won't lose much in mileage going from a v8 Grand cherokee to a Tahoe.