Here's some stuff to consider:
Custom work means longer down time and cutting new ground. There's always a risk that a mistake is made and problems happen. Conversely, bolt-on, developed parts engineered for a specific application by a reputable shop that has a solid track record of making good
systems likely won't have those "teething" problems a custom install will. I think this rule applies for most big changes one can make to any vehicle.
Supercharger vs turbo... the endless debate. It's purely my opinion that a supercharger would drive much nicer in a 4Runner. It will provide a linear power band with boost coming on at a predictable rate much like anyone would expect from a naturally aspirated motor.
A turbo, under the best circumstances, is sized either to deliver big power at the expense of low rpm responsiveness, or provides low end responsiveness but exhibits boost drop-off at higher rpms.
Usually, for most motors, this is the trade off made in any single turbo setup... and why lots of manufacturers and big name performance shops make insane bi-turbo setups.
Now, even with the perfect turbine/compressor size that gives a nice sweet power band that comes on early and pulls hard up into the sky; there's still oddities about a turbo setup that make it either totally awesome or kind of annoying and scary to drive: like boost threshold and turbo lag. Boost threshold is spot in the RPM band that under any particular throttle/load conditions, the compressor just isn't pushing out enough air to make boost. So, the engine can tend to be unresponsive and sort of a bit of a slug prior to this zone. Of course, once you spin up or load up the motor past this point and boost comes on, the party gets going...
Turbo lag is the time it takes between spinning up the turbine, generating boost, and the power to develop. The biggest variable (if I remember correctly) is turbine size in proportion to the displacement and the exhaust manifold size/length/shape. Essentially, it's how fast can the turbine get spun up to speed by the motor, and then how long does it take for that new found boost to arrive in the cylinders.
Driving a turbo car tends to be more visceral and organic. Lots of aftermarket kits or custom jobs can have some crazy personalities (and be alot of fun!). But, I would attest that in adverse conditions, I'd prefer a linear application of power.
All that said, a rear mount turbo takes all the downsides of a turbocharger and magnifies them. Granted that some people don't mind it for the payoff that they can literally choose their power numbers, however consider that predictable and linear power delivery is always easier to control.
:flamethrwoer2:
TOO LONG DID NOT READ:
Turbo Pros:
- Sounds Awesome
- Big power
- Chicks dig it
- Unique, custom job, envy of all
- Hot-hatch crushing power
- sounds AWESOME
- POWER!
Turbo Cons:
- UHG... LAAAAAAGGGG
- Expensive to fix
- Pre-turbo intake leak can cause tuning problems on an AFM setup
- Post-turbo intake leak can leave you stranded
- Difficult to tune (not easy, but not hard either)
- Compressor/turbine selection is an art and science
- Rear mount has LOOOOONG oil lines... exposing you to risk of engine loss in the event of a line failure.
- Rear mount setup should have a flex section. I read that a regular flex section would likely eventually fail under the pressure and heat induced by the turbine. Lack of a flex section could put alot of strain on the exhaust manifold studs.
- LAG...
- All the extra weight of intake piping
- The turbo itself is vulnerable to damage from road debris
Supercharger pros
- Bolt in
- Sounds ok
- linear power band
- predictable acquisition and installation costs
- path well traveled: no R&D budget needed.
Supercharger cons
- Not as sexy as turbo
- Not as cool as turbo
- Not as much power as turbo
- Not turbo whistle
- Not turbo awesome
- Not turbo
Conclusion:
To be honest, if the word "cost", to you, personally, is a concern; just back away slowly from the Turbocharger and don't look directly at it too long. Otherwise it'll seduce you with it's beauty... glory... and POWER.
The URD Supercharger is a proven power maker from a reputable outfit and gives good driving characteristics with reliable and safe tuning.
But who wants any of that crap when you could have a Turbo.
The bolt-on aspect of the URD for $6k honestly isn't all that bad. 
