Solace:
When we bought 'Imi Loa (Independence 31) almost 2 years ago we removed the existing marine head and holding tank and installed an Airhead Composting Toilet. The system isn't perfect but we are happy with our decision and our installation. The points brought up by newpbs are all goods ones to consider.
What we like about the system:
No holding tank taking up space (and no nastiness sloshing beneath the vberth)
Fewer thru hulls (well they are still there for now but will be removed/glassed in at next major haul out)
No trips to the pump out station
No holding tank smell (the boat used to smell like the blue chemical the PO but in the tank, a weird sweet smell). There is no smell in the boat, in the head there is a faint earthy smell that is not offensive at all.
Very simple to maintain mechanically
What we dislike:
Emptying the urine bottle (the manufacturer claims you will empty it every 2 days, we find we need to empty it every day). This requires being near facilities or dumping overboard. Depending on where you are this may be an issue.
Gnats: Twice during the summer we had gnats/fruit flies take up residence in the head. I find adding diatomaceous earth to the solids helps. We have only had the system 2 summers so we are still learning/experimenting.
Things we have learned:
Temperature is important: I see you are in Olympia, we are in Portland, Oregon so fairly similar climate. We find that even with our "mild" climate (not so mild with week up your way!) we need to run a space heater on low in the head when away from the boat in the winter or the composting process stops and things start getting septic.
Ventilation is important: We installed a Nicro day/nite plus vent for the toilet but found that during the gloom of winter the battery wasn't able to keep a charge between the rare sunny day. Our solution was to tie the vent into the house bank with a switch allowing us to run it off solar (when on the hook) but run it off the house system when at the dock.
A few other random thoughts:
We use COIR (coconut fiber) instead of peat. COIR is renewable, fairly cheap, and compact.
We chose Airhead over Nature's Head for a variety of reasons but a primary reason was the Nature's Head design requires you to tilt the whole upper part of the unit back (exposing the solids tank) to empty the liquid tank. This is more work and more unpleasant than the Airhead design. This also requires more clearance behind the unit which we didn't have.
We never used the traditional marine head so I can't provide a direct comparison but from the various accounts I've read I would much rather deal with our composting toilet. I am not familiar with the layout of the 35-2 but I wouldn't think our 31 would have a more spacious head compartment. The fit of the Airhead was just about perfect for us. It is definitely taller that a marine head but it sits on the little pedestal the marine head sat on and is fine. You can see a picture of the original head and the Airhead on our blog (scroll about half way down):
http://imiloa.wordpress.com/2010/06/15/progress/
For us the composting toilet was a no brainer but it all comes down to what your are comfortable with and what your expectations are. I would rather deal with the idiosyncrasies of microbes turning poo into soil than with holding tank chemicals, pump outs, and joker valves.
I'd be happy to answer any questions you have about our experiences with the Airhead.
Rowan