ARC VT-100MKII to MKIII, upgrade worth the cost?

I would leave it as MkII. I did some extensive comparisons between Mk II and III and actally liked the Mk II better. In my opinion the Mk II version is simply more musical with fuller, more palpable midrange. The Mk III seems dryier, brighter and sort of "mechanical".

Based on my experience, the Mk II version is actually fuller and more fleshed out then Mk III. To me Mk III was simply too dry and cold. The differences are subtle and I could easily live with any of them, but if I had to chose, I would go for Mk II.
I recently retubed mine with new 6550's and I'm glad I didn't order my tubes from ARC. The price they charge is simply ridiculous. For $200 you can get a tightly matched octet of Svetlana 6550s (same as ARC uses) from at least two reputable online tube dealers. The 6dj8/6992s are not expensive either and remember that they will last much longer than 2000 hours the 6550s are rated for.