How do you choose between 450 V vs. 750 V as a power source for your EV?
Both are in the same safety class, so must be robustly insulated, but an arc won't jump through dry clothing, so there is little to choose there.

750v needs thinner wires for the same power, but, at the scale of a car, this matters very little.

The main difference will be the availability of semi conductors, and motors. You will be using stock semiconductors, you may be using a stock or a custom motor. As the semiconductors get up over 1000v withstanding (you need the margin for transients) their resistance increases more than proportionately.

I would strongly suspect there is a greater variety of parts available for  450v applications than 750v applications. Though if those available for 750v are the right ones at the right price, that's not a problem. Given their size, support around the world need not be different due to availability either.

The rational way to choose between the two is to build a honkin' great spreadsheet listing costs and efficiencies of good designs at both voltages. Then you have to arbitraily choose some weighting factors to reduce all your efficiencies, weights, availabilties etc to a pseudo cost, so that you compare each solution as a pure cost comparison. Warning, choose different weightings, and you might get a different result.

The irrational, but far faster, way to compare the two is to start an outline design of each. If you come across a killer reason why one route is impossible, or so much better than the other, then use that reason to make your decision. If that doesn't happen within a week, then toss a coin.