How to choose best oscillator frequency?

[Squibcakes]

Wrong !

USB requires 48MHz for the USB-part, the core runs slower.

Hmm, I don't believe this statement is quite right either. Depending on your clock setup, you can have both the usb and core running at 48MHz.

Concidering you can drive the Osc inputs with a 4MHz xtal and pump it up to 48MHz thats value for money!!

I generally like using 20Mhz xtals because the body is small flat and rigid.

For non time critical (commercial) apps - I just use the internal RC osc.

When using Timers with interrupts etc- in general you can get longer delays when using a slower clock.

Squib