Aico all day for me. If you mix fire and CO on the same interlink, you should also include a test and locate switch.
The CO detectors have a feature called Audiolink that allows you to download a report from the detector using an app on your phone which you can email to the client, useful for checking if something might be going wrong that isn't a full alarm condition.
They've always been helpful on the phone and do offer free training when their training wagon is in your area.
The RadioLink modules are also very good, and whilst they might seem expensive, in some situations they will work out cheaper than running the interlink cable.