I currently own a 2001 100 series and am looking to downsize, because of horrible fuel mileage and a lack in the need for that much space. I am not married, college student, and would love something that is better on fuel. Not really interested in the 4runners with the 4.7, although it's a great engine...been there, done that. Interested in the 4.0 for power/fuel mileage, in the 4runner and the 2.7L in the Tacoma 4x4 Access Cab. How you would all compare the build quality/drivability of the two? I feel like the 4runner is almost class-less, whether dressed in a t-shirt and ballcap and going kayaking or on the way to a white collar job. It looks great and is fairly comfortable and powerful. How does the build quality differ, if they do? Where are they built? Is 20mpg possible in the 4runner 4x4 4.0? I've seen reports of 24mpg possible in the Tacoma 4x4 2.7, but if the 4.0 could do 20....that'd be something to think about. Is it really worth giving up my 100? I'm not much of an offroader anymore, although I love a good road trip (my 100 keeps me close to home because of dismal economy) and would like to keep 4x4 for snowboarding.
As Vantage said, it is definitely possible to exceed 20mpg in a 4th gen 4runner with the 4.0 and 5 speed combo (2005 and up.) Not sure about the economy of the 2003 - 2004 with the A340 4 speed. Now that's highway MPG. In the city, 17 - 19 is more realistic (remember, you're still talking about a body-on-frame SUV with a curb weight of 4300lb.) On relatively flat ground, if you keep your foot out of it, you can get 22mpg pretty easily.
What's even better is that with the large (23.5 gallon) fuel tank, the 4th gen has an impressive cruising range. Right now, for example, my 4runner has about 210 miles since the last fuelup and the gauge is just below half, with my Scangauge showing me a DTE (distance to empty) of about 190 miles. 400 miles on about half highway, half city driving is pretty decent for a vehicle of this size and wieght.
As for your other questions, the 4runner has always been built in Japan. Tacomas are either built in California or Texas, depending on the year (not sure what year they shut down the NUMMI plant in CA.) Currently Tacomas, Tundras and (I presume) Sequoias are built in Texas.
Having owned both a CA built Tacoma (04) and two Japanese 4runners (a 99 and my current 07) I think build quality is roughly the same, however it is worth noting that in past years there have been recalls on Tacomas for things like rust on the frames and now something to do with the cable suspending the rear tire.
The biggest difference between the two IMO is that from what I've seen, Tacomas tend to be more "stripped down" in terms of features (particularly the 4 cyl models) where most 4runners are "loaded up" with lots of features that, while not essential, are still "nice to have." Things like a 4 link rear suspension vs. the Tacoma's leaf springs, Multi-mode 4wd vs the Tacoma's conventional 4wd, a nicer sound system, fog lights, automatic HVAC controls (set the temp and the system does the rest), a trip computer (that actually doesn't work very well, but it's still kind of cool) better seats (though this is subjective and in truth I don't know much about the 2nd gen - 2005 and up Tacomas.) With both of my 4runners I find myself being constantly impressed by a lot of "little things" that just make it feel like a quality vehicle. Things like gas struts to hold up the hood instead of needing a prop rod (the last vehicle I owned that didn't need a prop rod was my '71 Blazer.)
Also, don't know if it matters to you but the 2005-up Tacomas have a composite/plastic bed. I haven't heard of any issues with this bed, but it might be something you'd want to know. I would imagine that if a panel was broken or cracked the only way to repair it would be to replace the entire panel.
For snow country, I have to say it's nice to have a rear window with both a defroster and a wiper. Obviously if you have a pickup, that's not an option even with a topper. And the 4runners pushbutton roll down rear window is nice for ventilation or for accessing gear in the back of the vehicle without opening the lift gate.
Now obviously, those "luxury" features are a double-edged sword. They're great when they work and a PITA when they don't. FWIW, even with 150k on it, I never had a lick of trouble with my '99 4runner, and at 100k I haven't had any issues with my '07, but the more electronic gadgets there are, the more potential failure points there are. I think this is more an issue that will be faced by the 3rd, 4th or 5th owners than it will be by the 1st or 2nd owner, but it may be a consideration if you're planning on buying a vehicle and then driving it to, say, Alaska and keeping it for 10 years. In a situation like that, the "simpler" vehicle may be better.
Overall I think the differences between the two are minimal and really come down to whether you need a truck or an SUV. I realized three years ago that while a pickup was great for a single guy, someone with a family has a harder time with it just because it's more difficult to carry passengers in a non-double-cab truck (mine was an '04 Extended cab.) I figured that my "truck" needs (transporting motorcycles, carrying big pieces of furniture, etc) could be adequately served with a simple utility trailer, and so far that's worked out well for me. Of course, I have the luxury of having a house with a big driveway and a big backyard where I can store a utility trailer - not everyone is in that situation.
Hope that helps!