It's a small, small matter, but when something has to be made both bold and italic, is there any reason to prefer
There is no difference in the font choice or italic correction applied and for common variants like bold and italic, it's probable that the fonts are preloaded so it makes no difference at all, however in principle
first loads the italic font (and will generate warnings and substitutions if this font is not available) and only then loads the bold font (after any font substitutions have happened.
of course is the reverse, and so loads the bold non-italic font as an intermediate step.
In normal usage this is no concern, but if using special font families only available in restricted variants it is possible to switch all the attributes without loading intermediate fonts
for example selects the default italic and bold attributes then selects the font specified for that combination. But then you need to do the italic correction manually.
I think there is no difference, since most modern fonts provide a unique shape for
Some programs like MsWord build the shape when it is missing, however the result is unsatisfactory. And of course this is not the way LaTeX does.