Combining the two would make more sense, meaning put the airport farther away where land is cheap and the flights won't annoy people and run passengers too and from via a high speed shuttle. Bonus you can put security on the pickup side of the rail line.
You'd need to back this up with strong zoning. I think it was Sacramento where they put the airport out in the countryside .. and then land developers discovered all this cheap land near it. Sure enough, the new residents started complaining about the noise from takeoffs and landings.
Flight paths is one of the things I look at when I pick a new place to live.
I find it annoying that they build commuter rail projects and scrupulously avoid going too close to the airport when by rights the airport aught to be a standard stop.
This is exactly what Hong Kong did: http://www.mtr.com.hk/en/customer/services/complom_checkin.h...