I’m not sure which generation of the Focusrite 18i20 you have but the converters on the 3rd generation are capable of 24-bit/192 kHz, whereas the Mackie Onyx 24 does 24-bit/96kHz.
That may not matter to you. As for myself I’ve gone back to my beloved Mackie Onyx Blackjack (1st gen) 2x2 audio interface which only does 24-bit/48kHz. But I love the way it sounds. 
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