You also have to consider the strength of the storm you are sheltering from. In most cases, you CAN ride out a tornado that is EF2 or below in a GOOD cinder/concrete block constructed building, or even a small interior room of a stick-built house. It is when the big ones hit that you need more substantial shelter, and being underground is safest. In the past, statistics show those tornadoes occur LESS THAN 1% OF THE TIME. But, as the weather has shown in the last few weeks, things seem to be changing. Your best bet is to listen to the LOCAL weather people, NOT a NOAA radio when the storm gets close. NOAA radios only repeat what is fed into them, whereas a local TV station is going to give you up to the second details of the storm. If you are lucky, the weathermen will be the caliber of those that broadcast in OKC, and have the same diagnostic equipment. (I know they DON'T have a clue OR the equipment here in DC - we are all just sitting ducks.)
As for above ground shelters.... I honestly would rather crawl into a culvert with the snakes before I hid from an F4 or 5 in an above ground shelter. There is no lab that can simulate a semi, or crude oil storage tank, being slammed into an above ground structure at +200mph. I don't give a rats butt that it can withstand a 2x4 at 300mph in a lab - there are LOTS bigger "missiles" in the real thing.
I was in Moore during the May 3rd 1999 tornado, and I have never stopped learning and educating myself about tornados since. The tornado spared our house with minor damage, but completely wiped our horse boarding facility at Tinker AFB off the map.