I have also been shooting both Leica and Nikon rangefinder cameras. I have a few M cameras, M3/M4s, and an M6TTL. On the Nikon side, I have the S, s few S2 bodies, a few S3 bodies, and an SP. No S4 yet, but I imagine I'll get one eventually.
For M lenses, I have a 21mm SA, 35/2 Cron, DR Summicron, and pre-aspherical Summilux. I generally shoot with a Nokton 50/1.5.
For S lenses, I have the 2.1cm O-Nikkor, 2.8cm f/3.5 3.5cm f/1.8, a few 50/1.4 Sonnar-type lenses, and a Millenium Nikkor.
I generally carry a camera with me everywhere I go. I am often carrying a briefcase or a backpack, as I commute to and from work from Tsukuda to Marunouchi on foot, and I often find interesting things to shoot.
I have found the Nikon S cameras much easier to handle when one's hands are full. The camera can be operated entirely with only the right hand, I can reach the aperture ring, shutter speed ring, and focusing wheel. I have to use two hands when shooting a Leica M, unless I have the camera pre-set. I find the life-size viewfinder in the Nikon S cameras to be much faster and easier to use than the finders in the Leica M cameras. I like the 35mm finder in the M4 and newer cameras, it takes a bit of practice to get used to the dual finder on the SP, or to use an accessory finder on the earlier cameras.
As for smoothness and silence of operation, it depends a lot on the particular cameras. I have a Millennium S3 which is far smoother and quieter than any of the 6 or 7 M cameras I have. Other Nikon cameras are less smooth, but all are relatively quiet.
The Leicas cannot touch the Nikon S cameras in terms of reliability. I have never owned a Leica M camera which has not required a repair or adjustment. I have never owned a Nikon rangefinder which has required a repair or adjustment. The Leica M is designed like a surgical instrument, the Nikon S is designed like a military weapon.
in regards to the lenses, it is hard to be objective, each has their unique strong points. My favorite lenses are the Nokton for the Leica, and the Millennium Nikkor for the Nikon. The 3.5cm Nikkor I had been using was quite scratched up, but still performed admirably. I have just replaced it with a clean lens, but have not shot anything with it yet. I love the Leica SA, but I haven't yet looked at the results from the O-Nikkor yet.
Anyway, I find myself shooting much more with Nikon RF gear than with Leica.