David-
4wd will definitely help you navigate your marsh in any situation (wet or dry), but it isn't a "magic bullet" and won't get you out of everything. (Another thread has a bunch of fellows complaining about 4wd - some of their statements I agree with, some I don't - but that's a different discussion....)
It sounds like your big concern is burying your tractor. Sinking is directly related to the "loading" of the tire contact patch with the land surface - e.g. weight vs. contact patch area, where "traction" is a much "stickier" subject. /w3tcompact/icons/eyes.gif /w3tcompact/icons/laugh.gif.
If you have a lower loading value, the more likely you are to "float" (although "float" in reality may really mean "sink more slowly" /w3tcompact/icons/sad.gif). Of course, there are 2 ways to lower this value - lower the vehicle weight (as we have been discussing) or increase contact patch (i.e. wider tires).
All else being equal (weight, tire surface area, tread pattern, tire material, etc.), if your marsh area is mucky on top but relatively solid underneath, then weight could actually work in your favor, allowing your tires can "cut through" and "grab" the layer of soil that would provide you more traction, allowing you to move through the area more easily and take "bigger bites" or pull implements more easily.
Again, all things being equal, if the mucky part of your marsh is 4 feet deep, a heavier tractor will sink you up to your axles faster than light tractor (it is still possible to sink a "light" tractor too).
If a 15 hp motor on "Tractor X" will "spin the tires", a 35 hp motor on the same tractor will not give you any additional benefit. This is far more likely to happen in a muddy/mucky situation because the coefficients of friction (both static and dynamic) are much lower than "normal" soil, hence you have less traction. The only time you'd see any real benefit of 35 hp over something lower would be where the HP can be effectively transferred to the ground - such as in a "normal" soil situation.
It really boils down to the surface area/weight value you want per your marsh situation. A wider tire will have more surface area, hence the loading per sq. in. of surface area will be lower. Conversely, a narrower tire will have a higher surface area loading. "Adjusting" (or really choosing) the weight of the tractor really just changes the other variable in the equation.
For all-out traction, you simply can't beat an Ag tire - but there are definite down sides (Ag tires will tear up the soil - don't even THINK about using them NEAR your lawn - /w3tcompact/icons/shocked.gif ) Since they are generally narrower than your construction or turf tires, they will sink in soft soil faster (bad if you want to float). They aren't for everyone - but something to consider if you want pure traction. After all, the "average" farmer uses Ag tires because he is doing activities that require putting a lot of HP to the ground. (Granted, some are wider than others, but they all have a pretty aggressive tread.)
Of course, all of the above is simplified and there are always exceptions, but it really boils down to (1) what type of marsh you have, and (2) what do you plan on doing in/on/around it.
