Electric Motor Efficiency

I am working on a project that requires maximum efficiency Electric Motors.

Is a 3 phase Motor more efficient than a single Phase? The 3 phase is less cost to purchase.

Under the same load, would it be best to use a higher H.P. Motor or slightly lower, in order to draw less Amps.?? I am using a 6 H.P. single phase now, but it is pulling 16 to 18 Amps. I tried a 5 H.P. many Months ago , and i believe it only pulled 11 Amps.

I order for me to use a 3 Phase Motor with 220 single phase House current, i had to purchase a $1,400.00 Beloit, Spin Master , it not only converts to 220 Single Phase , but allows me to decrease RPMs to an optimum desired speed. But $1,400.00 is out of the question cost wise.

Any help would be appreciated. Thank You