W. knows Miers. That is the difference.
The cartoon is completely accurate.
In the first case, Bush Sr. said "trust me!", because he in turn trusted the judgement of others who supposedly had inside knowledge and knew Souter so very well.
In the second case, Bush Jr. is saying "trust me!", saying we should trust his judgement for knowing Miers so very well.
Both cases involve us trusting the judgement of those with inside info, and both involve no objective indication of a history of conservative judicial philosophy. Bush could have chosen from dozens and dozens of candidates who have a proven documented history of conservative judicial philosophy latter criteria, but instead made a choice that reasonably fills people with doubt.
Souter, and about 3 other justices I can think of, ended up ruling contrary to how those with "inside info" thought they would rule. Those who vouched for them were absolutely convinced. They drifted over time. Such personal judgements are generally unreliable, it has nothing to do with Bush personally. The fear is entirely valid.