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".

I agree with Markxiii. I would add that the upgrade from MarkII to MarkIII is not nearly so dramatic as the upgrade from MarkI to MarkII. The presentation is a bit dryer and the breakin period is fairly long. YMMV.

One thing you do get with the MarkIII is a 12 volt trigger. This allows you to interconnect certain Audio Research preamps with the amp and to use the preamp remote to turn both the preamp and amp on or off.

It's also my understanding that the tubes are supposed to be longer lived and more reliable.

If you do decide to do the upgrade, summer time is the best time of the year turnaround time wise. The guys and gals at the AR plant aren't quite as busy.

Thank you for your feedback. Leonard at ARC told me the sound is fuller with MK III. Do you find it to be true? MK II sounds very good but a little light weight to my taste. Also, they charge a fortune for retubing and financially it almost makes more sense to upgrade., unless I go for tubes from other retailers.
Any thoughts?

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.