Comey's personal email versus Hillary's personal email
What's similar: both Comey and Hillary had classified information on their personal email. Both Comey and Hillary used their personal email for official business frequently, potentially raising record-keeping issues.
Comey's personal email was hosted on gmail, a commercial enterprise
Hillary's personal email was hosted by a small company and/or her personal employees
Legally, this difference doesn't matter much. Technically, gmail is likely to be more secure than Hillary's server.
Comey used personal email for some tasks but had a .gov address
Hillary used her server for everything and did not have a .gov address
This difference is important in theory but not in practice. Hillary's use of the personal server for everything means she was effectively evading record-keeping laws (probably her intended purpose) in addition to sending and receiving classified information. In theory, Comey's usage could be OK if he used the account only incidentally and did not use it for sensitive information, plus complied with FOIA and record-keeping laws. In practice, it appears he used it for classified information on a regular basis, so there's little real difference.
Broadly, Comey's usage fits the pattern of someone who wasn't as careful as he should have been with sensitive information. He could -- and arguably should -- be prosecuted for it. Hillary's personal server, on the other hand, demonstrates a broad intent to evade record-keeping laws in addition to carelessness with classified information and security issues in general. (Not discussed often is how easy it would be to forge an official-looking email from Clinton's server to the government server...)
It's possible for a fair-minded, objective prosecutor to exercise leniency in Comey's case. Yanking his security clearance, firing him, and ensuring he will not have a clearance in the future could be sufficient penalty, depending on exactly what information he was careless with.
In Hillary's case, there's just no room for that.
Notably, though, this new revelation about the contents of Comey's personal email strongly suggests that Comey and Hillary were vulnerable to exactly the same charge: classified information taken deliberately out of it's proper place and put in an unsecure environment. I've previously argued that Comey wasn't necessarily conflicted; he could have been using his personal email address within reason and within guidelines, without storing classified or sensitive information there.
It is now clear that is not the case, and his exoneration of Hillary set a precedent that now, if valid, would exonerate him. That seems a conflict of interest that should have warranted recusal, even if it was Comeys decision to make in the first place -- it should have been made at DOJ. Of course, Lynch was already recused and neither could risk an honest decision being made, because an honest decision would inevitably result in indictment for Hillary... and possibly, we now learn, Comey as well.