Isothermal vs adiabatic compression of an ideal gas
There are a number of ways to look at this. One is to look at the equations. Another is to plot the pressure p as a function of volume V. The area under the pressure curve on the plot is the total work done on the system. 



It is very easy to answer your second question by inspection of this figure, but perhaps not the first question without actually evaluating integrals (areas).

Yet another way to answer this, but perhaps hand-wavy, is to think of what happens as you compress the ideal gas either adiabatically or isothermally. In the adiabatic compression no energy is lost to the surroundings as heat. All of the energy transferred to the gas as compression work is retained by the gas. But the only way to increase the internal energy of an ideal gas is to raise its temperature. Therefore the effect of the adiabatic work is to increase the temperature of the gas. The increase in temperature is accompanied by an increase in pressure above that due to Boyle's law. This follows from 
$$p=\frac{nRT}{V}$$ 
In the adiabatic process p increases with the higher temperature above the level it would have if the heat had been lost to keep the temperature constant, as it would in an isothermal process. In that way the adiabatic compression is a more efficient way of increasing the pressure. This means less work is required to arrive at the same final p. By the same logic, if you want to achieve a determined final volume, you will have to work harder in an adiabatic setting because the pressure you are working against increases more rapidly as the gas gets hotter.