Hello, MT4 AND MT5 are the ones that I Know, currently I feel More confortable with MT5, in user experience feels More intuitive (like most of new technologies). You are using Demo so try all of them, se which one suits more for you.
Oanda, Pepperstone, Hugosway, etc are the ‘brokers’,MT4, MT5, Ctrader, etc are the ‘platforms’ used to trade. So before you decide whether you want to use MT4 (most popular), or another platform, you have to decide which broker is most suitable to you.
Yes MT5 platform currently become more popular, already having so many improvements for this platform if compared with MT4, but seems still not all broker support this platform, MT4 also still used many brokers as default platform, tradingview is good chart, and used many analyst to share their ideas, this is also a good platform