According to David S. Cohen, in a May 24th article at rollingstone.com, invoking the 25th would be "a steep hill to climb" and impeachment seems far more doable.

rollingstone.com


From the article:

"Why am I so confident the 25th Amendment will never be used, and really should cease being a part of the conversation? Because as hard as it is to imagine 42 Republicans voting to remove Donald Trump from office via impeachment, it's downright impossible to imagine 123 doing so. (Remember that the VP and the majority of the cabinet would have to vote against Trump twice. So that's at least 132 Republican votes against Trump.) To put an even finer point on it, if we were anywhere approaching 123 Republicans abandoning Trump, they'd just impeach him rather than go through the more difficult process. Even if the Democrats do well in the 2018 midterm elections, the number of Republicans needed to use the 25th Amendment will always be greater than the number needed for impeachment; it will be a steep hill to climb throughout Trump's time in office.

So by all means, please keep thinking about how to remove this dangerous, erratic, obstructive man from the White House. It's our patriotic duty to do so. But we must stop talking about the 25th Amendment as a serious option – because it's not one."

Impeachment and Senate trial may be the only viable path for his removal because he's not likely to resign in disgrace, a la Nixon, and give up the power of Presidential pardon that he may need to get Don Jr. off the hook and to take care of anyone who might turn on him to avoid jail time (e.g. Manafort and Flynn) and apparently he's even examined the possibilities of pardoning himself (after which he could step down). One thing is certain, this guy is scared shitless about what these various investigations might turn up and is acting like a guilty person. I hate to see this taking place in my country but people could have educated themselves about Trump and gone a different way at the polls. Anyhoo, it couldn't happen to a nicer guy.