Coming in late, if I missed something, sorry! That's a lot of pages. 
I too prefer the 4K range, but with ~3, 4, and 5K available, choose what you like! Just me, since I like to save money when I can, ebay has a lot of them in the $4-5 range if you buy 20-25 at a time, and I'm sure that is where they all come from (china) anyway. Especially if you are good with the 5K+ range. The 5K's are cheaper than 4K's. I prefer the clear covers, I figure that way none of the light is getting "wasted" passing through an opaque cover. Light dispersion from 11ft up is very good with no diffuser. I'd argue every bit as good as the few fluorescents that still have working ballasts in my fixtures.
And while more outlets are better, if you intend to line your walls to the max as I have with spare doors, tailgates, camping gear, workbench, shelf units for parts, etc., you'll probably end up blocking a few of them. For a welder have you considered making an extension cord? 110V is a no brainer to put them every six feet or so, but more than one 220V would seem overkill unless you plan to leave something plugged in all the time. Never compared the cost on the flexible cable (and you have 250ft of other, so may not make sense to even consider) since I already had it, but with one outlet where the bay doors open, I can theoretically move the welder (or plasma, whatever) 25' out into the driveway, or nearly reach the furthest distance inside the shop.

I too prefer the 4K range, but with ~3, 4, and 5K available, choose what you like! Just me, since I like to save money when I can, ebay has a lot of them in the $4-5 range if you buy 20-25 at a time, and I'm sure that is where they all come from (china) anyway. Especially if you are good with the 5K+ range. The 5K's are cheaper than 4K's. I prefer the clear covers, I figure that way none of the light is getting "wasted" passing through an opaque cover. Light dispersion from 11ft up is very good with no diffuser. I'd argue every bit as good as the few fluorescents that still have working ballasts in my fixtures.
And while more outlets are better, if you intend to line your walls to the max as I have with spare doors, tailgates, camping gear, workbench, shelf units for parts, etc., you'll probably end up blocking a few of them. For a welder have you considered making an extension cord? 110V is a no brainer to put them every six feet or so, but more than one 220V would seem overkill unless you plan to leave something plugged in all the time. Never compared the cost on the flexible cable (and you have 250ft of other, so may not make sense to even consider) since I already had it, but with one outlet where the bay doors open, I can theoretically move the welder (or plasma, whatever) 25' out into the driveway, or nearly reach the furthest distance inside the shop.

