Like O36 and O45 turnouts denote the different radii at which the divergent track leaves the main, #5 and #7.5 turnouts denote the ratio at which the divergent track deviates from the main. So the larger the number, the more slowly it departs from the main. In the case of the #5 turnout, for every 5 units you move along the main, the divergent track is 1 unit to the side of the main. And being they are more straight than curved you can run your train along them faster.
Swapping out O-72s for #7.5 switches is an entirely different issue than what is the difference between #5 and #7.5 switches. AtlasO #5 switches are somewhat equivalent to an O-100 switch, making the #5s much better operationally and aesthetically than an O-72. Replacing O-72s with #5s for a crossover greatly lessons the S curve effect.
If you have room for #7.5s that is great. But #5s are plenty good for 3-rail operations and are cheaper. And as Hot Water stated, the rail gaps in #7.5 switches may cause some of your engines to stop. This is less likely with a #5.
Late model #5 Atlas 3 rail switches are dependable. I found the Atlas 7.5 three rail switches to be great looking but offered less than desirable overall performance. One large 3 rail public RR had to switch out all their #7.5 for #5 switches because of dependable operation issues.
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