Which pair to choose depends both on how much shooting you do in fairly dim available light, and on how accurately you are able to focus in dim available light. The 35/2 and 75/2 are both excellent lenses. The 75/2 offers an excellent balance of high optical quality, compact size, convenient handling, and reasonable weight, and is excellent for informal available light portraiture. If you don't do much shooting in dim available light, or have difficulty focusing in dim light, the 35/2 and 75/2 will provide all of the speed and quality that you need for most of your shooting. If you do a great deal of shooting in dim available light and really need the extra f/stop to capture sufficient light to make usable images at hand-holdable speeds, the extra size, weight and cost of the 35/1.4 and 75/1.4 might be justified, however.