Hello.
We are a small entertainment trio performing commercial pop-rock music in local bars and events. We wish to get rid of all our analog gear and go all-digital to be as light as possible. The idea is to have a multi-channel USB interface to which we would plug 3 microphones, a keyboard (over MIDI) a guitar and a bass guitar, use VST instruments and effects and mix it all inside Sonar. The drums would be played via MIDI track and VST instrument. 
This way we could automate absolutely everything (volume, patch changes...) including the lights over an USB DMX controller and make individual headphone mixes for each of us.
For this we would need a 8 in 8 out interface.
Which of these would be the best for this purpouse?
We are looking for an interface with the LOWEST LATENCY through Sonar.
 
For our budget we have the options below:
- Tascam US-16x08
- M-Audio M-Track Eight
- Presonus Audiobox 1818VSL
- Focusrite Scarlett 18i20
- Roland UA-1010 Octa-Capture
 
We don't care about the onboard DSP, we wish to do everything inside DAW because of the automation.
 
(Please do not suggest other used gear, it has to be new).
 
Thank You!

I can only speak for the Octa-Capture as this is the only of the llisted that personally use ... but not live, though ...
 
The Octa-Capture can theoretically do all what you ask for ... and do it at really low latency given you have sufficient CPU power for what you intend to do ... however, I wouldn't put my life in its hand in a live situation as it occasionally has some quirks and hangs Sonar. It's rock solid doing only audio and direct monitoring, but e.g. Kontakt 5 is not its best friend ... so you would have to test, test, test, test , test, test, ... and hope it lasts on the road as it has not been designed for stage use, outdoors, beer thrown on stage, or likewise ...
 
Before you splash out - also have a look at the digital mixers out there that now come in 19" format ... they don't give you the low latency, but onboard DSP that's good enough for live gigs, USB audio, MIDI tracks for switchin FX and light, etc ... and they are designed for the road ... cost a bit more (maybe not that much), but should be more reliable than taking studio gear on the raod ...

Thanks for the response, first person experience with the interfaces, that's what I'm looking for - how reliable are they and what latencies to expect.
 
Yes, I've looked at the rackmount digital mixers also. The Soundcraft Ui and the Behringer X Air are interesting, but they lack advanced automation so they don't fit our needs.

If you'll be doing ALL mixing/processing in realtime via software, you want to be SURE your audio interface offers the lowest possible round-trip latency.  Otherwise, the mix/response will sound sluggish.
Of the audio interfaces you listed, the Presonus AudioBox 1818 VSL offers the lowest round-trip latency (4.9ms at a 64-sample ASIO buffer size/44.1k). 

I personally would think that what you need is a digital mixer, not an audio interface. I could see many issues with working that way. There are so many digital mixers now that your wish list is easily accommodated without worrying about latency and computers.
QSC Q mix, Behringer X air series  are worth digging into. . I think all major brands are going this route. 
You can certainly automate all the paremeters of these mixer, so not sure what you mean by that. 
 
None of those interfaces you listed would work well enough compared to a digital mixer. Most have very pour performance in the RTL department. 
I think you'd only get what you want with RME or similar priced units. 

Here is my scenario:
- we have 3 microphones that need gate, EQ and compression. Presonus offers this already with internal DSP so it would free the laptop's CPU from these tasks, which usually are set and forget.
- guitar would go direct in, using the VST Guitar Rig or similar
- bass would go direct in, using VST Ampeg SVX or similar
- keys would be played with a master MIDI keyboard using VST instruments
- drums would be played with a MIDI file using VST EZ Drummer os similar, or simply by a pre-recorded audio track to free CPU.
 
Now, by doing this inside a DAW you could play a song, that would automate volumes, effects settings like delays and reverb, patch changes in Guitar Rig and keyboards's VST and run a light show over a USB DMX controller, that has a VST plugin for it. And sure, all of us can have independent headphone mixes, again all automated. And all this synchronised to a playing song!
 
So all you would need is a small PA, a laptop, a 2H rack with the audio interface and a headphone preamp. It would then sound, feel and look like we have the whole crew of stage technicians to do it. No more chunky heavy gear, how nice would that be?

It sounds so easy when you can write it down in a couple of paragraphs.
But you have to imagine the overhead that all of this creates when all done in real time rrunning 3 or 4 VSTs simultaneously. You'll need to have a pretty heavy duty PC to do this reliably. Add to that the audio from 3 mics etc. If you get a chance to try some gear before committing to buy it would be good because you'll want to try it out in the studio before taking it on the road.

I would suggest that Presonus Geraldton is likely in your price range and they have an excellent forum for Live work. Post you questions there too. I'm sure you will get responses that will help you.

I don't want to detract from the guys here but we don't have hardware and integrated software like presonus does and I've done a lot of work with a Studiolive 2442 and hot a lot of help from reading the Live section of their forum.