for sus you want a 1 channel, you will get most power for your money
also each sub will get the same exact signal whereas if you use a twochannel...and hook a sub toeach channel, each sub COULD play differently depending on the song.
The primary benefit of a mono amp IMO is efficiency. Many mono amps are Class D amps and require less current from your car's charging system than a similarly powered 2 ch amp.
SOME mono amps are not class D and are really no more beneficial than a bridged 2 ch.