You will be very happy with the Trailmaster 6" lift.
The Rancho is also a very good lift. The old Rancho lift wasn`t to good, but they have reworked it and now it is as good as any lift. And from what I understand, you will get 6" or very close to that.
I have the trailmaster 6" lift, and it is extreamly strong. Very well built. Looks like it will hold up as long as any truck.there is atleast 5 people on this forum that have the Trailmaster 6" lift on the 88-98 style trucks.
The Rancho is now a one peice sub-frame. Which I don`t think it makes the lift stronger, just easier to install. But others have their own opinions.