If this vehicle is mostly for road trips and you plan to load it with your entire family + gear, I'd go w/ the V8 since you might need the additional power for passing at high altitude. I have the LX470, and I had no problems whatsoever passing other vehicles in uphill passing lanes at 10K ft between Durango and Ouray, CO. It just breezes by with a slight V8 roar.
Since the 4Runner weighs 900 lbs less than my LX, you should be able to fly up it! I've only driven the V6 runner (SR5 4WD trim) for a week in FL (flat, sea level). Driving was primarily rural @ 50-60mph. I got 20mpg on 2 tanks driven over 1 week, 87 octane, some driving/idling in 4WD on soft dirt/hard pack sand.
In my LX, I average 18.5-19mpg on the interstate/highways @ 69mph on cruise running chevron 91, so at a constant speed, I think it's a wash. For 100% city driving, the LX gets 15-16 mpg. I suspect you should be able to get 17mpg in the 4runner V8 due to less weight provided you're easy on the accelerator. There is a noticeable drop in fuel economy in the LX if I run 87...drop is around 1.5 mpg. That's a 10% drop while 91 costs only 5% more than 87, so you might also think about fuel economy this way.
OTOH, if you plan to use it as a DD, I'd get the V6, as you'll need every mpg you can get.
Some additional items:
1) In regards to the timing chain vs. belt, the belt on the V8 is spec'd to be changed at 90K intervals. Timing chains are usually much longer, like 150-200K mi (or whenever they start making that clanking sound which doesn't go away after a few seconds after startup). On my 22re, a timing chain replacment cost is $1.3K at the dealer. T-belt prices are typically around $500 (you do it twice as often, but it costs half as much). Water pumps typically start weeping around 120K mi, so it's a toss up. On my 4runner, I did the t-chain at 150K as PM since it was my only vehicle and I planned to keep it for many years.
2) Spark plugs: on the V6, it's recommended to change spark plugs every 30K mi. The V8 uses iridium plugs, which last 120K mi.
The 2UZ V8 is very easy on oil. My capacity is 8 qts (I reckon the 4runner would be a tad less), but the oil color doesn't darken easily like on the 4-bangers I have. Oil consumption is nil even over the 7.5K OCI's I run. If you search the landcruiser 100 forum on ih8mud.com, you'll find that the 2UZ doesn't consume oil and used oil analysis numbers are excellent. Another advantage of the V8 is it's been proven over 10 yrs in the Landcruiser, Tundra, Sequoia, and 4Runner.
