A quick backstory, I've had the C/Y 35-70 for some time now, and it is excellent. I don't use it all the time, but I always love the results when I do. I've known it had a permanent spot on my shelf, and Fred Miranda's comparison to the new Sony GM 24-70 where the legacy Contax was near as good everywhere but wide open, just reinforced that.
Marc Petzold on DPReview gets the credit for pushing the Minolta 35-70 f/3.5. Since I like knowing how lenses fair, I had to compare. I'll cut right to the chase, the Minolta is excellent, and has the nifty ability to be altered to open to f/2.8 at 35mm, though the IQ does take a hit. Let's get right to the results.
At 35mm the lenses are tough to distinguish in the center. The Contax is a hair warmer, and may carry a bit more contrast. It appears the light changed slightly between the two shots, based on the trees in the background. The Minolta may even be a hair sharper at f/11 based on the red sign on Fishy Fishy.
In the mid frame the Contax has a bit more glow wide open, but sharpens up quicker than the Minolta. At f/8-f/11 I think the Contax is better but you have to pixel peep to see any difference.
In the corners, we see similar results to the mid frame. The Contax sharpens up much quicker, at f/5.6 its plenty good. At f/8 the Minolta is acceptable though f/11 is better. It never catches the Contax but it's still a good and very useable result.
In the centers we have much the same results as at 35mm. The Contax is a bit warmer, maybe a bit more contrast. I think I like the Minolta a bit better wide open. By f/8 I couldn't pick which is which in a blind test.
Into the midframes, the Minolta is sharper wide open. The Contax catches up at f/5.6 and maybe just barely surpasses the Minolta. At f/8-f/11 I think I just barely prefer the Contax but they are both great and again I couldn't pick one from the other.
Yet again in the corners the Minolta has a slight advantage wide open, the Contax as some SA going on it seems. And at f/5.6 it again catches and supasses the Minolta slightly. I can the restaurant's covered porch has a corrugated roof on the Contax, not quite on the Minolta. Again at f/8 and f/11 they are very close. I think the Contax is carrying contrast better into the corner than the Minolta is, but both are quite good.
To my eye the centers are pretty much identical, with the Minolta possibly having a slight edge wide open but even that's debatable. Both are great.
The midframes flip flop wide open, the Contax looks stronger wide open with the Minolta not catching up until f/8 and possibly better at f/11? I'm splitting hairs again, they are very, very similar.
Neither corner is very good wide open. At f/5.6 I think the Minolta actually cleans up a bit quicker, I think the Contax still has some astigmatism at f/5.6. Both look great by f/8 but the Minolta is still a bit sharper.
I decided to lump these together rather than a write up for each focal length separately. I shot this to get a sense for mid range sharpness as well as how it handled bokeh in the mid ranges. There's really not a whole lot to highlight as far as distances go. I think the Contax is sharper earlier at 35mm and has more contrast, with the Minolta a bit better at f/11 Background is a bit smoother but Contax is a touch faster, both are pleasant.
At 50mm to my eye the Minolta is sharper and neither bokeh looks as smooth wide open with the Contax looking a touch busier. Flip a coin. At 70mm I again think the Minolta is a bit better, and both bokeh's are nervous. At f/5.6 and smaller they're hard to distinguish. Very similar performances here.
What test would be complete without looking at Bokeh?
At 70mm the extra 0.1 of f stop helps and the Contax looks smoother in addition to having slightly more blur. The Minolta just looks more nervous to me. At 35mm I think they are closer with both looking sllightly nervous but not bad for a 35mm. The Minolta does have a trick up its sleeve. Removing two small tabs held in place by 4 screws, you can take out the aperture limiter that lets the lens open to f/2.8 at 35mm. As you can see though, you get a lot more vignette and the bokeh gets more nervous. If you really need 2/3 a stop it's there but to me it was a pretty large hit to IQ. SA increases and effects sharpness a good bit.
If I didn't already own a Contax I'd probably buy a Minolta and be happy, but owning the Contax it isn't going anywhere. Personally I like the one touch zooms, and found the twist zoom mechanism of the Minolta a bit awkward to use. But I know others that have said they much prefer the two touch.
One other potentially important difference, both of these lenses offer an approx. 1:2.5 macro setting. However the Contax offers this at 35mm while the Minolta offers it at 70mm. For me, I have a Tamron 90mm f/2.5 so I prefer the 35mm option if I want something wider. Others might prefer having a more traditional working distance.
Both are great lenses, and it seems the Minolta's values are creeping up. Certainly if you're looking for a 2x standard zoom either of these should keep you happy.