Though these two lenses are very different, they have three things closely in common. The first, most obviously is focal length. They translate into an equivalent full frame field of view of 45mm for the Sigma and 52mm for the Voigtlander, so essentially both are standard primes. Secondly, they are also almost exactly the same size when the Voigtlander is fitted with the M to NEX adapter, though the full metal construction and larger amounts of glass mean the Voigtlander is heavier. Lastly, sharpness is quite similar over the overlapping aperture ranges - ie f2.8 to f16.
However these lenses are also very different. The most obvious difference is that the Sigma is fully electronically coupled with an inbuilt AF motor. The Voigtlander is old school manual. The second obvious difference is that the Voightlander is a full two stops faster. People pay exponential dollars for extra speed, and two stops is a long way. The third is minimum focal distance - the Sigma focuses right down to 30cm, while the Voigtlander stops at 70cm, like most rangefinder lenses. With a telescoping NEX to M adapter, the Voigtlander can be made to focus closer. Without this, I am pretty sure the background blur of the Sigma at f2.8 and Voigtlander at f1.4 would be very similar at their respective minimum focus distances. Fourthly the single coating on the Voigtlander produces different results to the multi coated Sigma. The Voigtlander has less contrast and a smoother balance to the colours - they don't necessarily 'pop'. It's kinda like comparing a water colour to an oil colour painting - both are beautiful, the medium is just a little different.
Lastly, they feel completely different. The Voigtlander feels like it is hewn from solid brass, with real glass, beautifully damped focussing and precise aperture control. The Sigma is just another 'plastic fantastic'. This therefore reflects in pricing. The Voigtlander is almost 3 times the price, but as mentioned speed costs money.