Stephen Gandy at cameraquest.com carries the new Zeiss Planar and the C/V Nokton, and is very familiar with the various Leica lenses as well. Since he has two horses in the race and a healthy respect for the third, ask him for a comparison.
BTW, the Zeiss Planar is built by C/V to Zeiss designs and specs, and under Zeiss quality control.
You could also find a used 40mm Summicron-C at a good price. Been using one for nearly 30 years and have had people claim 11x14s from it had to be MF with slow film and the right developer. The 40mm f:2 Rokkor M is the same lens, at KEH for $300 now in excellent condition. Looks like the 40 Summicrons have taken a big jump lately. Use to be about $250 in vg to excellent condition, now looks like closer to $350-$400 KEH bargain condition. But sometimes KEH bargains are in better condition than you bargained for. (Sorry 'bout that. ;))
In practical terms, you probably couldn't go wrong with any of the lenses mentioned so far. I also like the 1/3 stops, especially for transparencies. Money, features, angle of view, prestige nameplate... Nice to have choices. Oh, and the Planar has a cute blue index button for mounting orientation, entirely different from the Leica red button.
BTW, if you find a 40.5mm Minolta lens cap for the 40mm Rokkor M out near Crested Butte, it's my wife's. PM me here. ;)