Two paragraphs could be comprised of the same words, using the same font size, and fitting into similar spaces, yet look and feel quite different. Set one in a serif and the other in a sans, and the distinction is immediate. The point here is not to determine one is preferable. The point is to observe the effect of each on a passage of text.
A serif is distinguished from a sans by its small strokes finishing the ends of the larger strokes. This is useful, but it does not tell you whether a paragraph is legible or readable. Other factors, such as the x-height, stroke contrast, letter width, spacing, and individual characters’ shapes contribute to the effect of any font on a passage of text. A serif could be airy, open, or crowded; a sans serif could be neutral, geometric, narrow, soft, or technical.
To begin, create two text boxes of the same width. Paste the same passage of text (5 to 6 lines) in both, but set the left one with a serif, and the right one with a sans. Match them by sight. The same font size will look very different depending on the x-height (two 12-pt fonts will not necessarily look the same). Ensure that your line lengths are the same, that your leading, alignment, and paragraph spacing are identical.
With both versions displayed at the size at which you plan to read them, read the text. Do not focus on which version looks more “designing.” Do notice whether it is easy to recognize individual words. Do notice whether any version looks crowded or if the line leading feels too loose or too tight. Do observe whether your eye can easily hop from one line to another. Look for any letter or punctuation that looks like it has been overlooked. A letterface may appear beautiful in a headline, but may not read well when used in the body.
You might also try adjusting the leading differently to suit each version. The serif may require less or more leading depending on its ascender and descender heights and stroke contrast, and the sans may appear darker if its strokes are heavier and more even. Don’t feel pressured to equal the leading of one typeface to the other in an attempt to create a fair comparison. What you want to know is, after adjustment, which font works better.
Finally, consider context. Your text may be destined for print or for a screen or for a caption or an article. You may want to see your example set at the end in your final medium (e.g., PDF or print). Whichever typeface does the best job of allowing the reader to easily read the content is your winner.