I don't have a price differential, but the gypcrete way is the way to go if you can afford it. That's the way we are going. The radiant flooring is something my wife was passionate about in the design.

The good news is that neither heat nor cold have an effect on wood movement. The bad news is that moisture content does. Is this a genuine wood floor or one of the manufactured floors? If manufactured, I'd consult the manufacturer's literature. If natural wood, each species shrinks and gains on an individual basis. If plywood, with a veneer, the movement is less. But that would be a manufactured floor, which the manufacturer would be able to answer that. If genuine wood, your floor would be constantly moving, just like genuine wood moves in your house from summer to winter. If the wood had a Summer moisture content of 11% in the East when installed, and was reduced to 6% during the Winter, you would get shrinkages of possibly 1/8" per foot of width. If installed in the dry Winter, the wood would expand in the Summer unless your house was perfectly climate controlled, resulting in buckling. Wood improperly secured that doesn't allow for movement will eventually crack. Properly secured, it should fare well. Since heat dries the air, and dry air causes wood to shrink, your floor might move around. I'll have to ask around about this at work and get some other opinions.

I have a very similar situation - live in San Francisco, have full access to my basement ceiling to do the first floor - so I'll chime in on this thread and ask a few additional questions: - I can't get under the floor on my 2nd storey, so wonder if I can use the same hot water to run baseboard heaters up there (it only needs supplemental heat). - Any suggestions on how to improve air circulation/filtering if I discard my forced hot air system? - what is the ballpark cost for on-demand gas boiler, valves, and lines for a 1300 sq. ft space?

You can run hydronic radiant with only a 1/2" rise in floor height - you just need some plywood with channels for the tubing, such as QuickTrak (http://www.pexsupply.com/Quik-Trak-Radiant-Heat-System-819000) - I have a bit over 2000sqft in my house and it's fantastic. (I'm not affiliated with any manufacturer or vendor.) The 1-1/2" suggestion likely came from someone looking to float concrete, which is also a valid approach, but not necessary. It gets you thermal mass which has positives and negatives; I wouldn't worry about it. The WarmBoard mentioned above is the same approach as the QuickTrak (subfloor+radiant-on-top) except that it is its own subfloor, whereas QuickTrak goes over your existing subfloor. All that said, I think the biggest concern for you is your slab. If it's not insulated well underneath you'll just be heating mother earth and it's all a waste of time and energy, no matter whether it's electric or hydronic. And finally, on a pedantic note, electric radiant _is_ more efficient because it's a nearly 100% energy-to-heat conversion, by definition; however, the cost per unit of energy is generally significantly lower for gas versus electricity (1:2 in the Pacific Northwest for example), so hydronic off a gas-fired boiler is going to be more _cost-effective_.