No it is not. Best execution is a lot more complex than market vs limit. The normal metric for best execution is "implementation shortfall" which is the difference between the market price at the "decision time" (when the order arrives at RH is the best they can do here) and the executed price.

Poor execution leads to a lot of shortfall, good execution leads to a small amount of shortfall and in some cases even negative shortfall.

You can think about execution slippage as being the "information impact" of trading and a good broker will attempt to minimise this cost along with other transaction costs (eg fees etc).