The tl;dr: for most people the 18i8 is just fine and the 18i20 doesn’t really add anything. Consider the 18i20 only if you have a couple of fussy needs.
The main reasons I got it are because the 18i8 is a non-rack-friendly size and doesn’t support word clock generation for my offboard ADAT, meaning the ADAT had to be the clock source, which limited me in two major ways:
- Changing my recording sample rate required either flipping a physical, hard-to-reach switch on the back of the ADAT box (which is, as implied, in an audio rack so not very accessible) or mucking with my clock source settings and losing my ADAT ports, which makes switching between 44.1KHz and 48KHz enormously difficult
- Having my 18i8 operate at all required having my ADAT be turned on as well, whether I needed it or not, unless I wanted to change the 18i8’s sync configuration every time (which was annoying and easy to forget to do)
There’s a few other nice things worth considering on the 18i20; there are 8 VU meters on the front (for all of the built-in inputs) rather than the four “halo” rings on the 18i8, it supports some additional ADAT channels if you really need a crapton of inputs (or you can chain both ADAT ports together to get 96KHz support, which is… extremely pointless), and it also supports ADAT output, in case you need extra output ports for a bunch of effect sends or the like.
The 18i20 also gives you a preamp (including 48V) for all 8 of the built-in inputs, rather than only on the first four. Both models only support high-impedance inputs on inputs 1 and 2.
One big difference to the 18i20 compared to the 18i8 is the 18i20 only puts inputs 1-2 on the front panel, with 3-8 on the back panel. At first I was annoyed by this but then I came to realize that this actually gives me a more useful port configuration for my setup (particularly for my drum mics and my eventual Osmose), and the whole reason I have the ADAT box is to get more 48V-switchable front-panel inputs anyway.
I actually slightly prefer the 18i8’s halo VU monitoring, because it’s a lot more obvious which inputs are active or clipping, but it’s just a minor preferential change, I think.
The software is the same, the audio quality is the same, and the inputs are mostly the same. The 18i20 also changes the input ordering a little bit, which is very minor but now it’s a bit easier to remember that ADAT 1-8 maps to channels 11-18 rather than 13-20.
The 18i20 also has a built-in talkback mic that lets a recording engineer speak directly to performers over the monitoring outputs. I have absolutely no use for this feature, but it’s nice to have if you’re running an old-school professional recording studio, I suppose.
In any case, this is just a small upgrade for me, capability-wise, and if you’re interested in doing home recording and aren’t fussy about rackmounting your gear like I am, there’s practically no difference between the 18i8 and 18i20.
Also, one pet peeve about the 18i20 is that the rackmount ears are set back a little bit from the front of it, so the 18i20 does not fit flush into my rack. Fortunately I’m not that fussy. But it’s slightly annoying, all the same.
As far as ADAT boxes go, I have a Behringer ADA8200. It’s fine. I can’t really recommend buying Behringer products for various reasons in general, but this seems to be the only reasonably-priced non-ridiculous ADAT on the market anymore. I bought mine used for $200 at a local gear shop, although the new price isn’t much more.
For home recording purposes I would definitely not recommend Focusrite’s OctoPre line, though, as they are quite expensive, put most or all of the input ports on the back, and the only advantage over the ADA8200 is that they go up to 96KHz which is, again, completely pointless for pretty much any music-making purpose. I can see them being worthwhile in a professional recording studio context where the back-panel inputs would likely be more useful (since you’re using a cable snake to route the inputs into the actual recording booth anyway), though.
The added inputs on the 18i20 and the ADA8200 mostly make my life easier when it comes to not needing to plug and unplug a bunch of stuff when switching between instruments. It’s a convenience, not a necessity, and while this upgrade is definitely worthwhile for me, I wouldn’t say it’s something that most home recording musicians should think about until they have a specific need for it.