I think you should go with the 18i20. I think it's a fairly new interface from Focusrite, and it looks good too! Be sure to let us know how it goes after purchase. With USB, I just worry about other peripherals connected to the bus...e.g. a MIDI interface, a keyboard, a mouse, a USB-MIDI keyboard controller, a USB drive, an external USB HDD, a can cooler...you get the point. All things considered, those do take up USB bandwidth which
could have the chance to affect your audio stream or cause latency issues.
Overall, Focusrite is a good company, IMO. They will actually respond to problems and provide help. Just remember the time difference between the US and the UK!