Compared side-by-side, Aptos looks rounder and less square, and it has more natural looking curves.

The problem is that differences in fonts are kind of subtle when their goals are very similar. Both Roboto and Aptos seem like they're trying to be high x-height grotesque sans serif fonts. They're going to look similar to some other font, because there are so many in that category already.

(However, the capitals of Aptos remind me more of Gill Sans than of Helvetica, so I guess it's not strictly grotesque.)

Capital Q looks similar to capital O - like an O with large dust speck on your monitor. Since Q isn't common, I guess the chances for confusion are also rare.

Lowercase g looks weird since other letters with circular strokes have a lower baseline (bcdeopq).

The savings in total width by Aptos is very surprising

Ah! That'll be why I instantly preferred San Francisco, the widest typeface.