Good idea ...
I think that what matters is to open your demo account in the same place you're going to have your real-money account later on, so that by the time you switch from one to the other, you'll already be familiar with both the broker and the interface, and therefore not have to acclimatise to those at the same time as using real money for the first time. So the time to do all the "due diligence" is right at the start.
I trade mostly with Interactive Brokers, whom I strongly recommend (but I'm not sure how suitable they are for beginners, because they don't like to do a lot of "hand-holding" and I know that some people find their customer service when it comes to answering questions slightly less than optimal - but that's probably their only imperfection: on every other criterion, they're great.)
If you need a counterparty market-maker "broker" (these are not genuine brokers, but are what very many of this forum's members use), I suggest having a look at Oanda. They're ethical and honest, in my opinion (and I've done a lot of trading there in the past), and their "FxTrade" interface is really easy to learn to use. (I've also heard, more recently, that their spreads may not be quite what they used to be, so that might be their imperfection.)
I advise you to use a well-regulated broker, not a lightly-regulated "offshore" one, and to stay away from brokers who offer "deposit bonuses" because those are a really bad sign.
I also advise you always to check a broker's regulatory history carefully, before opening an account there: this is one of most helpful things you can do. You might want to avoid any brokerage that has a long history of adverse regulatory rulings and fines arising over issues related to how they've treated their customers in the past (there are some who are really recidivistic, and they can be well-represented in trading forums, too!).
Personally, I'd also advise you to stay away from any broker where you'd have to use the "MetaTrader" interface, because it's a clunky, horrible, unreliable piece of junk that also displays many indicators incorrectly.