MT5 Strategy Tester Spread
Variable Spread based on Data from the Broker. When using Real Tick Data, then Real Spread on a Tick by Tick basis is used.
It is "better" if you consider that you are using your own brokers data and not imported data from another broker. However, how your broker's data quality compares, is a totally different story!
I am unaware of such information as such comparison is against the forum rules (namely, not comparing or talking about specific brokers in particular).
Testing in Tester
MetaTrader 5 Help - Trading Platform — User Manual
https://www.mql5.com/en/ articles/2612
check this summary thread: All (not yet) about Strategy Tester, Optimization and Cloud
NOTE1: Broker/Tester spread (this thread title) is entirely different from "currencies correlated with each other e.g. EURUSD/USDCHF" spread that you are probably referring to. Same word "spread" - completely different meanings.
NOTE2: the good news: MT5 Tester design is intuitive and fairly easy to navigate - but you’ll still need to invest your time, read/explore carefully - there is NO WAY to skip learning curves.
- www.metatrader5.com