Natural gas units are the way to go - you'll need a slab (installed to code) and if I am not mistaken, most gas companies will hook the generator up for free.

You'll need to do regular tests, oil changes and transfer switch tests. An electrician needs to install the transfer switch, as your hooking it up directly to the utility line to your home. In most cases, the utility needs to pull your main breaker for the electrician to do his thing.

All in all, a bit of a pain to install, but well worth it. Just keep in mind that if power is out in your area, and your home is the only one lit up - you will be playing Holiday Inn for a while...