Difference between one HP single phase AC electric motor and one HP
three phase AC electric motor in terms of load bearing capacity.

Three phase power contains rotational information in the magnetic
field. Single phase has to create it artificially with capacitors,
etc. Two motors rated for the same load should differ only in size and
cost, the 3-phase model being smaller, cheaper, and more efficient.
Searching Google for 3-phase motor, I found:
http://www.team.net/www/shop-talk/hm3phase.html
This just describe a technique to use one with single-phase power. If
3-phase is power available, it is the only way to go. If 3-phase is a
future possibility, then follow these suggestions. If 3-phase is not
possible in the near future, then buy a single-phase motor meeting the
requirements for the job.

While chris is correct, he may be over selling three phase. Even in a
new instalation, initial cost will typically be higher, and a retrofit
rarely is good ecconomics. The 3 phase motors are cheaper in large
quantities and in the larger sizes, but may be more costly in
fractional horse power sizes and some shaft speeds, especially for
small orders. Local availability is typically better for single phase
motors. If smaller and lighter is not a significant advantage, you
need calculate if electric bill savings are enough to cover the extra
inital cost in 5 or 10 years. You may pay higher wages to 3 phase
electricans. DC motos have been finding more applications especially
in very small sizes and for electric automobiles.
 As cris suggested either way you buy a motor that will do the job
reliably. Single phase over ten horse power may not be available at
all; three phase motors are available to thousands of horse power.
Baring life will typically be longer for the 3 phase motor vs a single
phase motor as they are typicaly made better and have less vibration.
You are less likely to have to buy extra running horse power with a 3
phase motor to get enough starting torque. Typically there is less
motor noise with a three phase motor, which may make it difficult to
determine if the motor is turning.  Neil