Solid Hardwood, Engineered and Laminate Flooring - Confused about subflooring needed for parquet

Dear all: We just bought a condo and we want to install hardwood floors in the living room and dining room. The area is approximately 500 square feet. I had two different people come out to give me an estimate and the first one told me that since the floor underneath the carpet was cement slabs, that I would need to install plywood before installing the hardwood floors. This was especially true if we wanted thick (3/4") floors, since the planks would have to be nailed to the plywood. The second man told me that I should not choose such a thick floor because additional work would be needed to level it with the tile in the bathrooms and kitchen and the carpet in the bedrooms. He said if I choose a thinner prefinished solid oak floor, that it can be glued directly onto the cement slabs, and that it will last at least 8 years in good contidion, and as many as 15 years. He said I could even resand it and refinish it if I want to. His estimate was much more attractive, of course, but I don't know if he's telling me the truth with regards to the "glue on the cement" business... If this is true, then I will hire him for the job. I would also like some tips on what I should be looking out for when they're installing the floor, since we plan to be there as much as we can. We don't want a sloppy job, but we don't know what's right and wrong when installing a floor. Thank you so, so much for answering all my questions!
Desperate in Los Angeles!

Just did that to my basement floor. Was a poured cement slab, treated it since was in basement, then used liquid nails (perhaps my downfall, should have used subfloor adheasive for cement, and bigger trowel) It holds pretty good, but I went one step further and nailed the ply to the cement with 1 1/2 fluetted masonary nails (tip: unless you can accuratly swing a sledge, pre drill the cement about an inch so you dont chip or crack your floor) 12 nails per sheet, this stuff aint never comming up. (hehe) I then glued parque 12"x12" squares onto the ply (used correct adheasive for parque) Floor is only 1/2" higher then the hallway and garage (1/4" ply, I think the parque is 3/8 so a little more then 1/2" Looks good though.