Sorry for my slow reply- I'm on the road to CES. First, Warrenh, the output level will be the same, but it will be easier for a tube amp to make more power into the high impedance than into a low impedance, and the amp will sound better too.
Wireless200, what is given up is the power that a transistor amp might be able to make into the lower impedance. So if sound pressure is your goal, you want the lower impedance. If sound *quality* is your goal, the higher impedance will be preferred. If you are running tubes, that higher impedance will offer better sound and output power at the same time, without tradeoff.
These days there just is not the profound argument for 4 ohms or less anymore, since even transistor amps sound better driving higher impedances.