People have had great experiences with the Sienna AWD system in the slippery and snowy conditions. Its a FULL TIME AWD System so all wheels will be moving at the same time but when a wheel slips, it will send power to the wheels with most grip.
The Sienna FWD models w/ Traction Control and Vehicle Stability Control work well in wet and mild conditions. It sure helps the FWD drivetrain move up those hills with ease but still there will be slipping (Skid Indicator light will indicate this)
If you were to keep the vehicle for a long time, i'd buy new. Also you get the benefit of breaking in the vehicle with proper driving for the first few clicks. And the 07-08 Models have that new 3.5 Litre V6 in it which really makes this van move quick off its feet.
One thing i'd worry about in the AWD models is the Run Flat tires. They seem to bald quickly (some have only have them for 40000 KM!!!)
In the Lower Mainland (Where I live), There is only one so you will have to base it on what they are selling it for, $24600 for a 2006 Sienna LE AWD with 26744 KMs.
What a surprising difference eh?
Good luck Sienna Hunting
Tim
