Now I've read the whole thread, looks like we repeated the discussion on box v rsj from last January lol. JazzCNC is right in that either will do and arguably the rsj is the simpler to construct with, but lets examine the 'weightier' issue of a 40kg gantry (32kg rsj + other parts) vs a 20kg gantry (12kg box + other parts). You say, from earlier, that you plan on using 16mm screws x 2, each 800mm long driven by 2 slaved motors @ 3Nm each on 48v.

A rough calculation suggests that would be good for:

rsj: 9m/min cutting and 16m/min rapids
box: 11m/min cutting and 20m/min rapids

given the same acceleration. If the former speeds are good enough (and I think they are IMHO) then the choice is down to personal preference; either will work.

There is no need to increase the driver voltage, upping it from 48v to 60v has a marginal effect (increases the rapids on the rsj version to 18m/min, doesn't justify the extra expense IMHO).

One other consideration. the lower mass of the box version would allow one motor to drive both X screws at approx 6.4m/min cutting and 11.5m/min rapids @ 48v. Again upping the voltage has relatively little effect, increases rapids to 13.3m.min. If these speeds are acceptable, then this could mean buying one less driver and stepper motor and a smaller PSU (but adding pulleys/belts to the cost). So you need to factor this as well.