While I know Don at Skyguard is working toward certification for 2020 compliance, it's not clear what the path will be for folks who buy his current systems to get to a 2020-compliant system down the road. It's not going to be as simple as him emailing a form for you to put in your logbook--I expect there will be more stringent antenna installation requirements at a minimum, and perhaps hardware and software changes as well. There are already 2020-compliant "out" solutions that are available for not much more than one of his non-compliant systems costs today, and with those you know exactly what you are getting.
Once you have the "out" part, then your "in" options are very easy to compare. Pretty much you have three choices: 978 (UAT) only, 1090-only, or dual 978/1090. These days, there's no reason to get anything other than a dual-freq "in" system. You can build a dual-freq Stratux for a little over $100 that works perfectly fine. No one else comes close to that price.
So: If you want 2020 compliance, buy a 2020-compliant system, and get a Stratux.
If you aren't ready to buy a 2020-compliant system, get a Stratux today and take what traffic you can get, knowing it's not as good as you could have, and get a 2020-compliant system when you're ready.
If you're not ready to buy a 2020-compliant solution and cost is no object, then a SkyguardTWX is a good option for now, but expect more expense down the road to eventually transition to a 2020-compliant system.