I think the point was also that Harold fostered an antagonistic relationship by literally never telling Dillinger anything about himself. Even if it always would have worked out badly or Dillinger would never have been as good as Reese, Reese was never in a position where he had to bug Finch and Finch told him about the Machine in the very first episode which was long before he could be sure that Reese could be trusted with it. Dillinger was a tool but Finch made a lot of mistakes with him. Dillinger said that literally all he knew about Finch was his taste in tea (and he still got that wrong) while Finch threw out snippets to Reese far earlier.
Harold at the end said 'trust me' but gave him Fusco and Carter levels of information and even at the very end when it was the only chance to stop Dillinger from killing him and turning over the Machine to people who would misuse it Finch still couldn't give him a single reason why (not that I'd blame him for hesitating to open up when a betrayal was in process).
It's rather brilliant. The whole incident with Dillinger taught Finch to both be more open with his partner and to be more careful with who he selected, and as a result he not only chose Reese but decided to tell him about the Machine pretty much outright.
Absolutely agreed. This episode really connected the dots on multiple levels, not just plot-wise but character-wise. I don't know how much Finch investigated Dillinger prior to hiring him, but from the latest episode I assume he mostly focused on his military/tactical skills. I don't know if Finch had considered all the implications of a "hired gun", namely that he would be hiring a literal mercenary who could turn on him if a better offer came along. And I also don't think Finch initially considered the moral aspect of it all (and what else is new?). Ingram was his moral compass and he died; after that loss Finch reconsidered a lot of things. And, as we saw, Dillinger's behaviour likely also affected Finch. Seeing someone being so callous and cavalier about people's lives didn't sit well with Harold, obviously (which is interesting in itself as it was Harold himself who earlier had no problem dismissing them as "irrelevant").
But Reese was a different story. Before last week's episod,e Finch knew of him, as we saw (and we still don't know exactly why), but didn't know him. Witnessing that final moment of Reese saving Casey's life, not for money but because he genuinely believed the guy was innocent, was a revelatory moment for Finch, I think. Michael Emerson showed it in his face. Surprise, maybe admiration even. In that moment, he must have thought "they don't all have to be like Dillinger". Reese, much as he declared it, never really was a killing machine, not indiscriminately. I do think that's why Finch actively tried to find him and then snatched him up as soon as possible, as we saw in the pilot.
When it comes to trust, I also agree with the above. I think Finch initially must have thought "I'm paying Dillinger, he'll ask no questions and do his job, he's a mercenary". Which isn't inaccurate. But, even regardless of professional curiosity, a mercenary is always looking for an angle, always looking for profit. Loyalty isn't really a factor. So, by hiring a mercenary and by keeping everything a secret except the elusive "numbers" (enough of a hint to set Dillinger off), Finch basically had the ingredients for a mess. Which is what happened.
Comversely, when Finch chose Reese, he made the decision to tell him about the Machine from the start because, I agree, he had learned from his past mistakes. But also because Reese earned that trust. By saving Casey, by showing he had a soul, by showing he didn't blindly follow orders (as we also saw with the Ordos mess, when he knew the hit on Stanton was fishy). So I absolutely agree that Finch learned those two lessons; one, that in order to earn loyalty you have to extend a certain level of trust, and two, that you should really be careful who you trust with even the tiniest bit of information. Reese, for all his faults, is both loyal and trustworthy. And with last week's episode I must say that I, as a viewer, got a new appreciation for the way Finch must feel about that.