Toyota V-6 has been the same 190hp, 210lb.ft. 3.4liter 5VZ-FE since '97(or maybe '95-96 iirc).
My '97 Tacoma uses the same drivetrain as the 4Runner you're considering. Toyota drivetrain is the workhorse of the world(notice Iraq footage, and you'll see some Datsuns, and very many toyota trucks with hoods missing, guns mounted in the back, etc) for many years. There is no question in my mind that Toyota beats Nissan for drivetrain ruggedness and durability(in this instance). I've driven, worked on, diagnosed problems with both models of nearly all years.
Toyota has over-engineered many parts of the drivetrain, suspension, chassis, and most important: BRAKES. While Nissan went through their state of flux, Toyota was thriving... building top notch, reliable, durable "small" trucks.
Since we're talking "used" here, fit/finish will be directly related to just how well(or poorly) the previous owner maintained the mechanicals and cared for the interior.
My quick horror story: At 58K, the crank pulley came loose, destroying the crank and causing some real problems. Luckily, the dealer warranted a new block, new crank and all conn/crank bearings, among many other misc parts, and all labor was covered "under warranty". I still can't get an admission that this problem with the crank was known within Toyota. They sure did give away 3 days of labor, and an unknown amount of money in BIG parts pretty quickly though.
I did end up paying for 60K service, water pump replacement, head gasket replacement, trans/diff service, timing belt replacement....proof that looking for the "extras" will bring in dollars, eventually.

but that's just shop talk...
Even after that, I'd still pick a Toyota over the Nissan in the years you're looking at. Build quality, overall, is better. I consider my engine trouble to be the exception to the rule of toyota reliability. I've talked with many Toyota owners in the years since the incident, none of which have experienced such problems..
If it's for the wife, I'm sure she'll be quite pleased with it's visibility range from driver position, ease of use for all controls, and basic amenities....nothing over-complicated here. It really is a simple machine...the simplicity is the beauty of it. Assuming no major problems like mine, the low overall cost of ownership, maintenance should keep you happy too. Oh, and most all DIY work is a snap...coming back to the simplicity of Toyota design of these model years.
Phil:rant