You've seen the different opinions on this thread, so there is little to add...
2 years ago I had the same question: 28-70 or 35-70? I had been happily using a Vivitar 28-105, but I realized I could afford Zeiss...
My main consideration was the range of focal lengths. The 35-70 may be wonderful, but if you like to shoot wider or longer, it just does not make sense. If truly most of your shooting is between 35 and 70, then go for it.
The point I am trying to make is that to me the focal length is the number 1 deciding factor. The long controversy re. the performance of the 28-70 is kind of boring. I got the 28-70 and I was a very happy user. If you want to convince yourself you want another lens, go to Photodo and see it has a very low mark. If you want to convince yourself it is a very nice lens, go to
http://www.geocities.com/ilprode/Test.htm
Later I found a bargain VS 28-85, and now it is the one I use, not so much for the quality (which is, as everybody has said, outstanding) as for the range--85 is much better than 70 for my shooting. Unfortunately, as more people has pointed out, flare IS a problem.
And now that I have 2 VS zooms (of which I don't need one, obviously) I would still prefer a 28-105...The reviews of the current Tamron are excellent, with quality (and price: list $1,491, available for $850 new!) at Zeiss level, they say. But I have never used one. The Vivitar Series 1 I used, still in production (list 330, available new for less than $150), was very good (or so I thought)-- at least the one I have (someone has complained about it in this forum, and the mark it got in Photodo was an abysmal 1.9! --one of the worst, go figure).
Bottom line: the VS are all good, you can't go wrong with them. My advice is to choose based on focal length. Regarding non-Zeiss short zooms, my only experience may not be representative.
Good luck,
Juan