I've owned both and definitely prefer the 45mm panacake (AI-P version, I've not tried the GN).
It's tiny yet with great handling, well built, sharp wide open, has wonderful bokeh - certainly my favourite out of any 35mm format lens I've ever owned. I've sold all my Nikon bodies but will be keeping this lens just in case they make a smaller FF digital in the future.
That said, some people out on the internet have owned it and didin't like it, so I guess it's subjective.
The Voigtlander is pretty good too - a stop faster (though generally I find 2.8 is fast enough, and when it isn't, f2 is still too slow) and the quality is pretty good too. I found it gave a surprisingly vintage look to images for a modern lens. In the end I found it to be really a bit large for a pancake lens, especially with the hood attached, though I had the first version which is a bit longer than the current model.
I reckon you can't go far wrong with either though.
A couple of samples from the 45mm AI-P:
And the Voigtlander 40mm: