Features both units share:
- 8 analog in- and outputs
- Mic-pre amps on all 8 inputs
- High impedance (Hi-Z) supported on first two inputs (meant for guitars and such)
- Digital in- and output (SPDIF, coaxial) for something like a CD-player
- Headphone output with volume knob
- MIDI in and out
- DSP mixer that is configured from the computer, but that can stand-alone (you can't change the levels though)
- Rack-mountable
Roland Octa-Capture
Pro / Additional features:
- Two DSP effects: reverb and compressor
- 192 KHz recording
- Auto-set of input levels (maybe a bit of a gimmick)
- Smaller unit (= more portable)
- Software looks easier to use/understand than the Focusrite software
- Includes better software, though that is a personal preference
Con:
- Goes down to 4 channels when recording 192 KHz sources
- No individual knobs for setting the input gain levels (means pushing a few buttons)
Focusrite 18i20
Pro / Additional features:
- Additional headphone output with volume knob
- ADAT in- and output (digital, mostly for connecting additional in- or output channels)
- Monitoring output for (powered) speakers; this way you don't loose two regular output channels
- Word clock output (an input might make more sense, I think) for synchronizing multiple audio interfaces
Con:
- Only records up to 96 KHz (shouldn't really be a limitation)
- No monitoring effects (such as reverb)
Personally I think the DSP effects of the Roland are of limited use; they're mostly useful for giving musicians a better idea of what they'll sound like when recorded. I'm assuming you'll record the source material without effects and will apply effects (if wanted) afterwards, especially since you can't change effects levels if you record a 'wet' (effects applied) signal and because the quality of the effects applied in a DAW will be of higher quality.
I like this picture from the 18i20 manual:
Note the use of a separate 4-channel headphone amplifier for the musicians headphones; you can do this both on the Roland and the Focusrite, but you'd still need a separate 4-channel headphone amp (or separate ones, of course). One of the following two Behringer ones listed below might fit the bill; they're not super high quality, but I don't think that's needed here and the headphone amps will have no impact on recording quality anyway. Generally Behringer stuff is fairly sturdy and has a good price-performance ratio, but you can definitely get much nicer stuff (for a much higher price).
http://www.behringer.nl/EN/Products/HA400.aspx
http://www.behringer.nl/EN/Products/AMP800.aspx
I think both the Roland and Focusrite (by default, but you may be able to change this) will output a combination of all input signals on the headphone outputs. If you use a regular output (in combination with a headphone amp), you get much more control over what sound you send to a headphone. I think one of the two manuals even describes the headphone output(s) on the front as meant for the 'sound technician'.
In general, I think the Roland looks a bit fancier and is probably more portable and maybe a bit easier to use. The Focusrite makes more sense in a studio setting and should be easier to integrate with other equipment. In then end, I think both units would make a great 8-channel audio interface and while both units can be used stand-alone as a stand-alone mixer (with fixed levels, although you can change gain levels) you need to connect a computer (laptop?) to get optimal use out of them. But you'll likely connect a computer anyway, because otherwise you can't record anything and an 8+ channel mixer would make more sense.
