You can't predict amp "loudness" just by looking at the power rating. There's a lot more going on...
Differences in speaker frequency response and efficiency typically matter more than a doubling or halving of amp power. Ditto many specifics of the amp design: EQ will greatly affect perceived loudness. So will the size of the output transformer (in general, more iron will give you greater bandwidth - more lows and highs - at or near full power) and the design of the power supply (less sag will make the amp seem louder and punchier). Preamp distortion adds compression and tends to make an amp sound quieter because you lose the note attack.
Even in the case where all things are equal except the output power, a 3 dB change can be very noticeable in the the context of a mix. Most people believe that 3 dB is a "just noticeable" change in volume. That figure was arrived at by having a lot of people try to differentiate the volume of two tones in isolation. And for that purpose, the 3 dB figure is probably useful. But sit down with a studio engineer or a mastering engineer someday and have him show you just how noticeable a 1 dB change in level or EQ can be within the context of a mix. The same thing applies on stage: 3 dB can make a difference between being buried in the mix vs. being out in front of the mix.