You have to take resistance into account. Slope, terrain, etc all add to the amount of force it will take to get you unstuck (hence the 1.5 GWV rule). Trust me when I say you will regret going cheap on a winch if/when it doesn't free you from whatever obstacle has ahold of you. Of course, you could get a snatchblock or two to multiple the pulling force and go with a smaller winch but that is your choice. The multimount is a good option as it allows you to free yourself forwards or backwards but it won't help if you do not have enough winch to get you out. Just my .02.
Taken from http://www.pirate4x4.com/tech/billavista/Recovery/
More importantly, the "effective weight" of a "stuck" 4x4 is very often FAR more than 1.5 times the GVW. The following data on how to more accurately estimate the "effective weight", is taken from the world of professional heavy recovery - the guys that recover Tractor-trailers that have flipped on their side for instance, as well as U.S., Canadian, and UK Military recovery manuals.
Once you have accurately estimated or measured the trucks loaded weight (LW) you can calculate the resistance to be overcome in any recovery situation (this is commonly known as the ROLLING resistance). There are 4 types of resistance that must be accounted for to accurately assess the resistance that must be overcome. These are surface resistance, damage resistance, mire (stuck) resistance and grade (slope) resistance. Calculate them all as follows:
A pull of 1/10 LW will cause a free wheeling truck to move on a hard, level surface.
A pull of 1/3 LW will cause a free wheeling truck to move on a softer surface, such as grass or gravel,
A pull of 2/3 LW will be required to move if the wheels cannot rotate (as if the brakes were fully applied), the pull required to overcome the resistance (drag) the truck id 2/3 or 67% of the LW. Damage resistance includes surface resistance (i.e. you only use one or the other)
Stuck (mire) resistance:
A pull of 100% of LW will be required if the truck is stuck to a depth of the sidewall on the tires.
A pull of 200% of LW will be required if the truck is stuck to the hubs.
A pull of 300% of LW will be required if the truck is stuck to the frame..
Mire resistance includes damage resistance (i.e. you only use one or the other)
Grade (slope) resistance:
Upgrade (vehicle has to be recovered up a slope or grade)
15 degrees - add 25% of LW
30 degrees - add 50% of LW
45 degrees - add 75% of LW
Vehicle recovery on level ground - no correction
Downgrade (vehicle has to be recovered down a slope or grade)
15 degrees - subtract 25% of LW
30 degrees - subtract 50% of LW
45 degrees - subtract 75% of LW