What are the differences and advantages/disadvantages.

And Offset vs Not Offset?

One advantage is a four coiled trap will come up through a few inches of snow. The two coiled will come up part way in slow motion.

Off set is better IMO. I held a muskrat by the front foot once in a bridger #2 offset. And less damage, better hold to the paws of critters.

And while I am at it, is Animal Traps and Supplies a good store or no?

Are you water trapping or land trapping. What are your target animals.

Minnesota Trapline Products Tim Caven owner they will be glad to help you out

Coyotes, land trapping.

JMO, but standard two coil is perfectly fine for all conditions save two.

Heavy frozen or crusted ground or snow and heavy modified jaws.

Consider the extra springs as extra lifting power or boost not holding power. A well designed trap gets it's hold or lockup from the interaction between the levers and jaws at the lever. Some are better than others in that regard and extra spring is meaningless if that lockup is off.

Since I might get frozen ground once or twice every other year and I ain't sticking extra steel on my jaws I'm sticking with two coils.

As for offset or not, frankly I don't see enough difference to matter. I'll use both but I buy closed jaw. The only argument for offset that holds water to me is the slight gap allows the levers to rise just a hair higher for a possible better lockup. But remember my some are better than others? If it's right to start with it doesn't need a hair more to lockup.

BTW, the above is for canine land traps. Beaver and coon are different feet and conditions.

Beaver, I want four coils as water and silt need more boost. NO OFFSET on any taper footed critter like beaver and raccoon front feet. Matter of fact I want traps for those to "bite" with excellent lockup.

Coons you want to consider a trap jaw system that contains or shields the foot. Double jaw or high pan.

In my opinion offsets came to be because of lower quality traps that needed higher lever position for better lock up. I measured some coyote feet in traps last year and then were all 3/8 or bigger between the jaws. So unless your offset is bigger than that they are all squeezing the foot theoretically the same. If u get a trap that doesn't squeeze the foot tight you will get more sliding of the foot between the jaws which seem to be harder on the foot from my experience. Plus I want to hold possums, skunks and raccoons when they come along too.

Four coil is faster and more power coming through tight soil but harder to release animals from if you need to release a catch.

I use offset four coils but I do run 1.75 traps for coyotes so I'm not sure if the bigger traps would need the four coil.

Since money is such an issue for you, if you're trapping for fur, I'd just go with closed jaw 2 coils since they will do all you need them to and are cheapest.

I like 2 coil offsets. Just my worthless 2 cents.