@Arthanor: As far as I know small radar does not have better detection chances. Quite opposite, small radar has 10% every 30 minutes, large radar 20% every 30 minutes.
The rest is true, they cost less, they are finished sooner and they stack, so they are definitely a good use of your money.
As for the "middle of the region", like most of the players, I don't know exactly where the region borders are exactly, so it is hard to find the middle. Also I don't know the UFO flying patterns precisely, so I cannot judge if the UFOs will fly close by or not.
IMHO, small radars are useful... but large radars are better.