If you possibly can, go w/ the pier and beam foundation. That way, if something breaks under the house, you can fix it. And things DO break in the lifetime of a house, don't let an overly optimistic contractor tell you different.
Slab foundations are like transistor radios- fine, as long as nothing breaks. But what do you do w/ a broken transistor radio? Send it off to get repaired, or toss it in the trash after you've bought a new one? You toss it. It's just not cost effective to get it repaired.
Same thing w/ slab. God help you if something breaks under the house- a pipe, a compromised vapor barrier (holes in the plastic between the bottom of the slab and the earth the slab rests on. Talk to people who have had stuff break under the slab. They'll tell you- it's astronomically expensive. Usually involves heavy duty equipment (large jackhammers, drills, pumps, concrete equipment, & lots of really expensive labor).
Why such a hassle? Because everthing under the slab rests on 4 inches of reinforced concrete! It couldn't be harder to get to- you literally have to bust up the slab, or dig holes deep enough for workers to walk BENEATH the slab- to make repairs.
What problems can develop, you say? After all, the slab is all ONE PIECE, right? Fine, if it stays one piece. See, lots of times, the slab starts cracking up, for a variety of reasons, many of them really out of your hands. If your slab becomes unstable due to drying out of the soil, erosion, poor drainage, or the land starts shifting over time, then very often, the slab starts to CRACK UP. Slowly, but inexorably. The more cracks develop, then the more it starts to move around- up and down, warping the frame, causing the network of sewer lines and water lines under the slab to buckle and fracture. Not uncommon for sewer line under a slab to break, spreading raw sewage under the house.
It costs the better part of a grand to repair ONE water line crack under a slab foundation. If the foundation deteriorates over enough time, it has to be corrected (usually, a temporary fix, in spite of what all the foundation-repair outits tell you). It costs many thousands of dollars to do this- 12,000 is common. The slab is `adjusted' - propped up w/ columns of some sort placed under it. Fine. Then, 5 years later, it could need this treatment again, for another huge outlay of your money. This scenario happens to a lot of people who have concrete-slab foundations.
Residential concrete slabs started becomming popular in the late 40's, as a COST CUTTING feature. Only, you can't rush concrete work- slabs have to be very carefully poured- over carefully chosen and prepared sights. You can bet your ass this is done for commercial buildings- lots of liability if a commercial building has a faulty foundation. But that's commercial construction- the big time, where things are usually done right, in deference to the lawsuits that big builders can usually bring to bear on a contractor that cuts corners. Residential concrete foundations are done as quickly as possible, usually assembly-line fashion, as a contractor develops a subdivision. Since it costs several times more to PIER a slab (rest it on top of columns of concrete poured into the earth beneath it to keep it as stable as possible over time), this is almost NEVER done for residential slabs. And any little screw-up that Joe Hardhat makes during the laying of the foundation will be buried under the slab, until it causes a major problem- a poorly laid pipe can bust loose and start to leak, a hole punched in the vapor barrier can cause your carpet to rot from water seeping up from the ground into your living room. This kind of stuff happens all the time- little screw-ups made at time of construction causing big (buried) problems years later.
In a nod to the havoc that faulty concrete slabs caused unlucky (and soon-to-go-into-debt) honmeowners, the construction industry developed a new and improved residential concrete slab in the early seventies (at least, that's when these slabs started gaining wide use. Basically, it replaced the rigid iron reinforcing rods embedded in the concrete with more fexible cables made of stranded wire. This cables made the slab more `flexible' than the rods- it would allow the slab to bend with the land, so to speak.
Of course, contractors saw these new-and-improved slabs as a cost cutting measure- they started building houses with them on land formerly thought too unstable for slab foundations. These were largely untested. And...they crack w/ the best of them.