Main difference is the availability of OCO "bracket" orders and Chart Trader (trade from the chart). These features are only available with the full subscription version.
The Free Version will include full charting and DOM, but you will have to manually manage any working orders.
NinjaTrader Direct (Free) includes the full charting including historical data.
If you do not use Chart Trader or any trading automation, then take advantage of using NinjaTrader for No Monthly Platform Fee.