I've been talking about these two cameras for the last two posts. Here I'd like to do a head to head -
Shutter speeds: The Leica has manually set shutter speeds of 16s to 1/8000s in half-stop values, plus B and X. The F4 has 4s to 1/8000 (8800 the top speed in auto). Since I like to use fast lenses, the 8000 top shutter speed is one of the most important selling features for both. Plus I like taking pictures of the sun. Seriously.
Lenses: both have excellent world class lenses. Nikon lenses are faster and cheaper and more available but they can't do the Leica thing. Nikon has many more F mount lenses than Leica R mount lenses available. The F4 has an incredible integration of ANY F mount lens ever. The R8 can use many of the R lenses but it's dangerous to use anything besides the later 3 cam and ROM versions because it can damage the ROM contacts on the R8/R9 body. The F4 has an advantage here but the R8 is still versatile.
AF: Nikon has it, Leica doesn't. Who cares
Modes: both have all the modes I need. In general regarding cameras, manual works fine, aperture priority is really nice, shutter priority is neat and everything else is "what exactly is that for?" The Leica has: manual, aperture priority, variable automatic program, Shutter priority, Prior-to-exposure flash metering. The Nikon has: Programmed auto-multi, shutter-priority auto, aperture priority auto and manual. If you are interested in program modes you might want to do some research as both of them have their unique specialties in this department.
Exposure: The Nikon has exposure lock, exp. comp and a double exposure lever control on the body. The Leica has all that AND a sophisticated system for clamping down the film and controlling exposure for multiple exposures. Hella cool
Winder and frames/sec: Both have removeable power winders, the Leica at 4 frames per second with a convenient battery pack and the Nikon (F4s) at 5.7 frames per second with 6 AA batteries. The Leica also has the option for a motor winder which is smaller, quieter and does 2 frames per second. Sometimes I wish my F4 had a motor winder option as it's nice not having to be distracted by winding and I'm not shooting at 6 frames/sec much.
Metering: both have multiple types including matrix and spot. I have heard the Nikon matrix metering works better.
Viewfinders: The Leica has a fixed viewfinder. The Nikon has several viewfinders including a waist level viewfinder. I'm a big fan of this option for several reasons - When using a tripod at max height, about 5', I have to bend way over, being 6'4". The WLF is also more discrete than pulling your huge F4 or R8 camera to your head and you can get great candid photos this way.
Mirror lockup and viewfinder shutter: both
Focusing Screens: many types available for both.
I've already hinted a few times about the tremendous price difference between these two. Here's a comparison of some lenses these two have in common (in N vs L form, used prices):
Body with power winder: $300 vs $800
35mm f/2: $100 vs $450
50mm f/2: $50-100 vs $300
50mm f/1.4: $150 vs $600
80-85mm f/1.4: $800 vs $1200
135mm f/2.8: $150 vs $250-300
180mm f/2.8: $300-400 vs $250-600
Why are the R8 and F4s my favorite pro 35mm cameras?:
This can be answered in 3 smaller questions -
Why Nikon and Leica? LENSES
Why 1/8000 second top shutter speed? OPTIONS
Why pro SLRs? CONTROL
Conclusion: if you need a professional 35mm SLR, the F4s is the obvious answer at $300 for a body (or an N90s for $80 if you're really on a budget) and a plethora of relatively cheap excellent lenses. If the most important thing is having a camera that suits you and your art, you'll have to consider the R8 too.