Is there a difference in military court proceedings that would make it easier to convict the "seditious" congress members?

TL;DR:
There's no civilian case against Mark Kelly because of the First Amendment protection of freedom of speech. There could be a really poor case against him in military court based on the more strict regulations of the military, but it would likely be thrown out due to lack of evidence. This means that it would be slightly easier to convict a (former) member of the military, but, in this case, there is no real case in either court system. In this sense, it's technically true that it's easier to convict Mark Kelly in a military court than a civilian one, in the same way that it's easier to fill a 1000 gallon container with 0.01 fluid ounces than with 0 fluid ounces.

Long form:
The First Amendment makes it clear that what Mark Kelly and other Congress members said wasn't illegal and they were simply using their First Amendment rights to free speech. There are exceptions to the First Amendment, but those don't apply. The statements made were reminding members of the military to not obey illegal orders, which is also reminding them to only obey legal orders. That, in itself, is completely legal and the way they said it is not misleading in any way.

The Uniform Code of Military Justice (UCMJ) Articles 90, 91, and 92 talk about disobeying legal orders. There could be a case that Mark Kelley was implying that Donald Trump's orders were illegal, but even in military court, it's nearly impossible to convict someone on the premise of assumption of implication. The military, especially the UCMJ, is all about regulations, laws, orders, and the Chain of Command, so the the Judge Advocate General (JAG) doesn't really give much credit to assumptions. Also, the military and JAG are supposed to be non-political and serve the Constitution, not a specific political party, even the party of the sitting POTUS. The military also has to judge the legality of the orders of the POTUS, so they can't be ordered to convict someone just because Trump says so. That, in itself, would be an illegal order. Generals and Admirals in the Joint Chiefs have already pushed back on previous orders from Trump, as being illegal orders, so there's precedent for doing exactly that.

As for what could be considered illegal orders from Trump that military service members may need to refuse, there's plenty of media coverage explaining how law professionals and professors explain how certain of Trump's orders are illegal. Much of that is contentious, so I'm not going into it.

The military presumes that all orders are legal, but the reality is that every member of the military has the right and even obligation to question every single order they receive. The UCMJ Articles 90-92 specifically talk about legal orders, but the other side of that coin is that any illegal order that is obeyed makes them subject to the rest of the UCMJ and even civilian laws. Said slightly differently: the articles 90-92 simply protect service members from being prosecuted for following legal orders, while the rest of the UCMJ applies when the orders followed are illegal, and the UCMJ doesn't protect service members from civilian laws. In fact, the JAG may prosecute a service member after the civilian courts prosecute them for civilian laws being broken, even if they are found not guilty, as the UCMJ retains the right to make their own judgements based on the UCMJ.

Even trying to use UCMJ Article 133 of "Conduct unbecoming an officer" would be a stretch, since the message was to disobey unlawful orders, which any officer should be reminding their people. There was no incitement to riot, there was no alcohol involved, or anything else that could be considered "unbecoming".

The word sedition has a very specific meaning:

: incitement of resistance to or insurrection against lawful authority

https://www.merriam-webster.com/dictionary/sedition

Again, the advice by Mark Kelly and others was to not obey illegal orders. Nothing was said about disobeying legal orders. This means there still is no real case against Mark Kelly, even if one is filed.

Further reading on the Chain of Command and the right to refuse an illegal order: https://politics.stackexchange.com/q/61732/32885 https://politics.stackexchange.com/q/61583/32885

Full disclosure: I have answers on both the above questions, which is how I know about them.

Conclusion: Since there could be a case filed against Mark Kelly in military court against him for "conduct unbecoming" and no valid case filed in civilian court, it's marginally "easier" to convict, except that no conviction has any legal merit in either case. Said differently, because no case can be filed in civilian court without it being immediately and summarily thrown out, but a case could be filed in military court which would at least be reviewed before being thrown out, so it's minutely "easier" to convict Mark Kelly in military court than in civilian court, even though it's not going to happen in either court.

The major differences are summarized here. There are differences in process, in who serves on the jury, and... most importantly - in punishment.

For example, UCMJ Art. 134 says this:

Though not specifically mentioned in this chapter, all disorders and neglects to the prejudice of good order and discipline in the armed forces, all conduct of a nature to bring discredit upon the armed forces, and crimes and offenses not capital, of which persons subject to this chapter may be guilty, shall be taken cognizance of by a general, special, or summary court-martial, according to the nature and degree of the offense, and shall be punished at the discretion of that court. As used in the preceding sentence, the term “crimes and offenses not capital” includes any conduct engaged in outside the United States, as defined in section 5 of title 18, that would constitute a crime or offense not capital if the conduct had been engaged in within the special maritime and territorial jurisdiction of the United States, as defined in section 7 of title 18.

That in addition to the fact that the criminal conviction would affect benefits (but I believe that may be true regardless of the jurisdiction).

Also worth highlighting that charges in a military court are brought by military personnel, which Trump may think could be ordered to do so while the DOJ (supposedly...) cannot.

There's also the political point: Trump has repeatedly and consistently been trying to insert the military into the civilian political discourse. This is just another attempt at this.

Besides the process differences, there are also differences in the offenses that can be charged. There are many violations of the UCMJ that would not be civilian crimes. Although the statement mentions a federal law that applies to everyone, they may hope to find additional violations applicable to retired military personnel (and maybe they'll go back and search his time on duty) during the investigation.