It can be tough decision to buy into the Leica system. Since Leica's are so expensive great care must be taken to choose the components that best fit with your existing system. Also many people these days are selling their DSLRs to get a smaller and more portable system. The Fuji and Olympus lines are very popular with shooters wishing to downsize their camera bags. Fuji has done quite well replicating the look and feel of the Leica system.
Most people coming to the Leica world typically budget for a body and one lens. The choice of the standard Leica M or the Monochrom is the first decision to be made. As most Leica shooters prefer B&W, the decision is either to by a color camera and convert to B&W, or to buy a dedicated monochrome camera. Leica is the only company that gives us the choice and I thank them for introducing the Monochrom camera. This decision would take a lot of time and is beyond the scope of this article. My last word is that before writing off the Leica Monochrom I suggest viewing prints from an image converted to monochrome and a print from the Monochrom camera. The difference is startling.
With Nikons and Canons we have a wide array of high quality lenses to choose from at a price range to fit most budgets. With Leica we have relatively few choices and the price runs from very expensive to very, very expensive. The choice is either 35mm or 50mm. The speed of the lens comes later and the decision is really dictated by cost. Are you willing to pay almost double for one more stop? That is a choice that is between you and your pocket book.
The first step in deciding is what is your shooting style currently like. Do you like to take portraits or see the bigger picture. If you are planning on being a street shooter like most Leica shooters, do you like to shoot details or interactions of people. If you don't like getting in close to people the longer working distance of the 50mm might be more comfortable. When post processing the images after you take them on a smartphone and they are frequently cropped you might be a candidate for the 50mm. Finding yourself moving in closer to fill the frame means a 35mm is in your future.
The 35mm is the king of shooting in close. Of course you have a wider field of view but you also have more depth of field. When the subject is closer more depth of field is very advantageous to keep all of the subject in focus. It also covers for focusing errors that occur because if the subject is moving and if your not experienced with manual focus, it can be difficult to get an in focus image. Even a slow walker can be difficult to shoot within ten feet with a 50mm because subject movement is magnified. The 35mm is great for shooting people interacting with each other or the environment. That being said 35mm portraits are often not that flattering. Unless you are careful with what is on the edges of the frame you can end up with wide angle distortion.
The 50mm is great for detail work. I don't mean close up or macro work but the details of life. Look at the work of Leica master Henri Cartier-Bresson. He was so great at framing the action so that all unneeded elements are cropped out of the frame. If you are more of a purest and don't like cropping the image after you take it, the 50mm is for you. Because of the wider view of the 35mm it is more difficult to keep unwanted elements out of the frame. Although I prefer longer lenses for portraits, the 50mm is great for environmental portraits. Again, refer to Cartier-Bressons book of portraits. The are among the best ever taken. Mathematicaly the 50mm is the view that corresponds to the normal view of the human eye. Some like the straight forward vision this lens gives, no distortion of a wide angle and no image compression of a telephoto.
To help choose between the two lenses the best metric might be just to look at your own work. Fire up Lightroom or your program of choice and look in the info panel to see what focal length you prefer with your favorite images that you might shoot over with a Leica. The results might even surprise you. It is best to buy a lens that matches how you shoot now, not how you think you want to shoot. If you are at the point of putting thousands of dollars down on a body and lens it is best to match up with your current style so that you will grow as an artist.