Flacco was definitely a better Playoff QB but generally the Playoffs is what you'd use separate two QBs who play at a similar level in the regular season. For example Brady and Manning are close as regular season QBs but it's Brady's Playoff performances that puts him ahead of Peyton. Similarly Kurt Warner was much better than Peyton in the Playoffs but you wouldn't say he was a better QB overall because Manning was better over a much larger sample size in the regular season.

It is also worth noting that Flacco struggled a lot early too but was bailed out more than Lamar. He went 3-2 in his first five Playoff games but had 1 TD to 6 INTs and a passer rating of 55.