That is a very tough question to answer. I believe it depends on your requirements as a user.
TradeStation is very nice in the sense that it is simple. Execution as well as datafeed's are fully integrated making this a one stop shop. Additionally, they will handle correcting things such as bad ticks, etc. The new version also has some pretty nice new functionality.
NinjaTrader is very flexible and will allow you to do far more than TradeStation. For most complex strategies NinjaTrader is probably the better way to go. However NinjaTrader reminds me of TradeStation 2000i in the sense that you have to handle your own data feeds from 3rd party providers. This can be somewhat of a pain from time to time. It is very convenient however that they allow multiple connections, so you aren't 100% reliant on their data.
There are a number of other factors that you may need to consider, but I can't tell you everything as every trader has unique needs.