Pinned refers to a shim/pin that is used to tighten the crank so it will not "spin" (go out of alignment). Once aligned, the pin is pushed in (wedged in) to hold the two moving parts tight. As you can tell I don't know the proper names for the individual crank components, but I've seen a spun crank (mine!) out of the bike and know what it looks like.
Those who prefer the pin say the heat generating in welding can move the aligned crank away from its aligned position (and once that happens it's permanent - welded slightly incorrect). Those who prefer to weld say a pinned crank can still slip.
Darkhorse is the only way to go: http://www.darkhorsecrankworks.com/
Here's another great link from Darkhorse: http://www.darkhorsecrankworks.com/pdfs/nospin.pdf
You have hit the target on the head, only difference here is is pinning better than welding or welding better than pinning or 6 of one half dozen of the other. I have heard both versions of the slipping and being welded wrong as the reasons, was just looking for anyone with personal experiences with either / and or if they had had any issues. Only thing I had heard and have not confirmed it was if the pinning slips it can be repinned, was told once you weld it your done if you have issues.

Since I am not "racing" it, I think it should be ok.
FWIW, I ended up having mine welded and trued etc. I did this more out I know if i dont i will be sorry (murphy's law) I previously had an issue with my S&S crank in the prior build of the 124. Its been problem free since the reman 2 + years ago. Figured the 450 dollars was good insurance.