There are many pros and cons to consider when choosing a trading platform, including the cost of data, commissions, platform license fees, support for retirement accounts, support for accounts outside of the United States, licensing fees, the responsiveness of their support team when you need help, the look and feel of the interface, compatibility with their hardware (e.g. if they use a Mac or if they prefer to use a tablet or mobile device), and many other factors.
The decision is also dependent on the asset classes that you want to trade. For example, a Futures trader may feel that the low margins that NinjaTrader offers on Futures are important and justify paying for data and the license fee that NinjaTrader requires for full access to their platform's features, while an option spread trader may find the ease of working with options and spreads in thinkorswim with its free data and a no-fee platform to be more important, though they may pay higher commissions and require larger margins.
Those who are trading in an IRA may also like the flexibility and ease of trading that TDAmeritrade offers. And platforms that require separate accounts to trade Futures vs Equities may impact those with smaller accounts as they try to maintain a balance above the 25K Pattern Day Trader limit.
TradeStation offers a great rebate deal. You can effectively get an intensive (or anything else Base Camp Trading offers) effectively for free through 20% rebated commissions over time. You can get more details on this program at https://www.tradestation.com/promo/basecamptrading/. As you can see, there are advantages and disadvantages to each platform.
So there's a lot to consider, and the choice is highly dependent on the trader's needs and preferences. This is why we have traders using NInjaTrader, thinkorswim, Tradestation, Sierra, eSignal, MultiCharts, Interactive Brokers, Infinity, MetaStock, etc., and it's why we support all of those platforms with our indicators to some degree. Some traders have accounts at multiple brokerages so they can take advantage of the best features that each offers. thinkorswim, Tradestation and NinjaTrader are probably the most popular platforms among our customers, but ultimately, the choice of platform is up to the trader to match their own personal needs and preferences.