Which test is better. Historical data testing or real time trading.

Real time trading has always been considered better than tests run by using historical data. Real time trading fixes a lot of issues with historical data testing, such as slippage and roll-over cost of Futures contracts. On the other hand one has to bear in mind, that today’s real time data, is tomorrow’s historical data. With this I want to point out, that if historical data cannot be used as the market conditions might change, real time trading also becomes none valid as they’re ultimately based on historical data.