To answer that question accurately, you need to determine how fast you want the machine to move, and how fast you want to accelerate to that speed, then do a bunch of math to determine what voltage you need to achieve those goals. Without an accurate torque curve chart, using the same drives you'll be using, you may need to make a few assumptions to get an answer.
Actually, 32 x square root of mH = absolute maximum voltage you should use.I have learnt today that - 32 x square root of mH = ideal voltage.
The ideal voltage would be the lowest voltage that will allow the machine to reach your performance goals. That voltage would use less energy, and your motors would run cooler.
If you need the maximum rpm and power that those motors can deliver, then you'll need to supply them with 120V. At 48V, you'll only be able to use about 40% of the motors top speed.
But, if your design only requires those motors to spin at 300-400rpm (just a guess here), then 48V might be perfectly adequate.