I have normally gotten 15.5-16 with 80% around town driving. Simply put, I think that is the absolute best you can get short of doing 50 on a country road for a full tank. No ac, incredibly easy on the gas. There's several threads on mud about the difference between the 2uz in the sequoia and the 2uz in the 100 series. Basically the 100 series is much more overbuilt as far sat he parts go, and honestly the whole land cruiser is going to be better built. The sequoia is also tuned to be faster off the line, the way most Americans like to drive, and there really is nothing in the aftermarket for them. Unless you want to try to adapt stuff for first gen tundras, which we used to have, and there is almost nothing for them. I would say go with the land cruiser. Hold out for a well priced one, I used to drive a first gen tundra(same as sequoia just with a pickup body) and the quality and feel of the land cruiser is much better. That being said, several times with the tundra I was able to do 23+ mpg on the freeway. Land cruiser is heavier, and that's because it has a stronger frame and axles.
Edit: I believe the post 2000 cruisers(atrac was introduced then) have a more aggressive atrac compared to the sequoia. Also, I've seen several 100 series with over 400,000 miles, and I think it is probably the most reliable engine toyota has made(22re might win when you factor ease of repair).