Different people will have different opinions on this one, here?s mine.
Leicas gather light better than Kahles, or anything else for that matter, but 42 mm will gather light better than 32 mm so performance at dawn and dusk with regards to brightness will be a wash. The Leicas will be sharper, but the difference will not be huge. To really experience the advantages of Leica, you should get the 10X42.