Partly plagiarized from the FAQ I co-wrote at the Audio Asylum:
"Bridging is the summing of two channels to give one higher output channel. Because of the summing however, the speaker impedance is seen as half of its normal value. In other words, an 8-ohm speaker becomes a 4-ohm speaker load, and a 4-ohm speaker becomes a 2-ohm speaker load. Speaker impedance ratings are nominal only. Actual impedance may dip to a much lower value through part of its range. When an amp's current load has been doubled due to bridging, it can often fail to provide the required amount of current into the load. Sonic effects include harshness in the midrange and highs, and thin bass. In almost all situations therefore, biamping with similar amps will result in better sound quality than bridging."
The other problem with bridging is that it cuts an amp's damping factor in half, resulting in looser control of the woofer and generally sloppier sound. So to answer the question, yes, bridging sacrifices some quality in exchange for a few dB's of extra level.
My advice is to buy one better amp, rather than two lesser quality amps, but "better" doesn't really have anything to do with higher watts.
