I have the Warn 15k on my pickup, and have done some really, really heavy pulls. The key attributes to the 15k, when compared with the 12k (off the top of my head, subject to updates that I'm unaware of) is a larger diameter drum (which equates to a lower "gear ratio"), a shorter cable (90' on the 15k vs. ~120' on the 12k?), and a heavier duty cable. Unless equipment has changed, the 15k must be run with steel cable, which I think is also a larger diameter than on the 12k, whereas synthetic line is an option for the 12k. The rule of thumb is 1.5x the weight of the vehicle you are pulling (assuming your drive wheels are providing little to no assistance) to move the vehicle and overcome resistance of mud/snow/rock obstacles. Thus, a 15k winch is perfect for a 10k lb van, especially if towing a trailer. On the other hand, make sure your bumper has the space to fit the 15k - it's a significantly larger unit than a 12k.
2009 E-350 Lopes 55