I don't use digital cameras, and I don't own any of them, but...
Just for curiosity, why do you want to trade the Ultron?
It's considered a better lens technically, a wonderful one, and it's aspherical, what can be seen with digital cameras more than with fast film... None of them were designed precisely for sensors as far as I know, the Nokton having the strongest barrel distortion in Voigtlander's 35's line.
The speed difference is basically nothing in the real world... IMO it has (the Nokton) a very, VERY beautiful transition from focused to defocused zones, though...
I use a Summaron stopped down for street just because it's very small, has zero distortion, and at f/8 on 400 film it's like using a V1 Summicron... Mandler just used computers to recalculate the 3.5 Summaron from the late 40's, and offered two versions of the same lens but with better corner performance for wider apertures and closer focus, those being the 2.8 Summaron and the V1 Summicron from the late 50's; and I also use a 35 CV Skopar (2.5) I bought before the Summaron (I'm weak for selling gear), for the same reasons, small size and low distortion... A great lens, the Skopar, by the way.
More modern 35's, optimized for digital, like aspherical Leicas, are far away in price, another world... Anyway, what's wrong with the Ultron? Is it horrible for digital? I'm here more for learning than for any other reason... Corrections are well received too!
Cheers,
Juan