+1 for never heard of a spool breaking either.
as morton said already - a full spool is basically just a whole carrier, consisting of a solid tube that you insert axles into, and a ring for the ring gear to bolt onto. It replaces your whole diff centre.
a mini spool is simply a block of steel with axle splines and holes through it, and it replaces the whole inner parts of your current diff centre (i.e. it does away with all the spider and side gears)
welding is a cheap way out - but it's hard to get 100% right. you need to clean the whole thing out properly and then get HEAPS of heat into it (need a decent size welder!) in order to get any penetration. Then you have to be careful to not weld too much to stop the axles going in (or out lol), and so that everything still operates as it should! - i guess you'd also want to be careful about warping anything with the excessive heat.
advantages of a full spool over a mini include:
- much lighter
- less parts, and therefore less likely to break stuff
- it's a hollow tube - so if you break an axle at the splines, you can just smash it out with a broom stick or something from the other side. (whereas with a mini spool, you have a pin going through the middle that stops you from doing this)
and that's about it
here's a pic of a full spool and a mini spool
full:
minispool installed inside the original diff center:
if you got the cash... go for a full spool.
if you don't have the cash for that, go for a mini spool.
if you're adventerous or don't have the cash for either of the above - crack out the welder and beer