Are there specific advantages or disadvantages of an XML sitemap over a TXT based sitemap?

There is no fundamental disadvantage to text-only sitemaps aside from what you've touched on. Setting the priority and last-modified date can affect the crawl frequency and on a large site with both frequently and infrequently changing content that can be very important.

XML sitemaps can also carry media information (e.g. video sitemap) and if you have that kind of site that may be important to you.

But if you have a smaller site with mostly set/static content there is probably very little functional difference between text and XML sitemaps.

Explicitly stating this information also increases the probability of a mistake -- some modification date not getting updated correctly

No more of a risk that somehow outputting the wrong URLs in the text version

This is not an answer, but a caveat for XML sitemap wizard users. I recently did a tech & SEO overview of a site rebuild where an XML sitemap was auto-generated by a wizard. She failed to review it and see that all the URL's were like **new.**site.com/page.asp - the dev server where the site was built!

Typing out a txt sitemap has the advantage of deliberate attention. In this age of auto-generated pages, there are many opportunities for auto-generated blunders. This site was only 40+ pages, but hitting links and copy/paste is a welcome break to debugging.