I mean, I know this isn't going to be helpful, but you should go with the stronger piece. It's a tough question since, on one hand, I would lean towards a thesis excerpt because it also demonstrates that you can do sustained, independent research. On the other hand, I'd hesistate to give them the first 20 pages just because you usually don't get to the meat of your argument and really start showing off what you can do in the first 1/4 of your thesis. Also keep in mind that a "research bent" doesn't necessarily mean they're looking to see if you can cite and basically cover a state-of-the-field. It also means original thinking, close analysis, etc. (especially in the humanities, where they're usually not looking for a specific set of methodological skills). I would look back at your choices and look at which of the two best demonstrates your ability to think critically and articulate your ideas well rather than where the citations come from. You might also consider that if you have a 80 page document, you might pull 20 pages together out of it (e.g. a condensed portion of your introduction/reading review, a strong close reading, and a rejiggered conclusion to tie it together) that demonstrates both a depth of analysis and a breadth of skill.